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Friday 10 August 2012
               Trigonometry
   Using all 3 trig ratios to find missing
   lengths


Outcomes

Must: Use label sides of triangles correctly

Should: Be able to do calculations involving trig
functions

Could: Use trig ratios to find missing lengths in
triangles.
Friday 10 August 2012
            Right-angled triangles
A right-angled triangle contains a right angle.


                            The longest side opposite
                            the right angle is called the
                            hypotenuse.
The opposite and adjacent sides
The two shorter sides of a right-angled triangle are named
with respect to one of the acute angles.

 The side opposite the
 marked angle is called
 the opposite side.


  The side between the                             x
  marked angle and the
  right angle is called
  the adjacent side.
Label the sides
Calculate the following ratios
Use your calculator to find the following to 3 significant figures.


 1) sin 79° = 0.982                2) cos 28° = 0.883

 3) tan 65° =    2.14              4) cos 11° = 0.982

 5) sin 34° = 0.559                6) tan 84° =    9.51

 7) tan 49° =    1.15              8) sin 62° = 0.883

 9) tan 6° = 0.105                 10) cos 56° = 0.559
The three trigonometric ratios
                           Opposite
O
     H
                  Sin θ =
                          Hypotenuse     SOH
      Y
P      P
P       O
O        T
                           Adjacent
S
I
          E
           N      Cos θ =
                          Hypotenuse     CAH
T           U
E            S
              E

           θ              Opposite
                  Tan θ =
                          Adjacent       TOA
    ADJACENT



           Remember: S O H C A H T O A
The sine ratio

                   the length of the opposite side
The ratio of                                       is the sine ratio.
                    the length of the hypotenuse

The value of the sine ratio depends on the size of the angles
in the triangle.

    O     H
    P
               Y
                   P
                                                   We say:
    P                  O
                           T
    O                          E                            opposite
    S
                                   N
                                       U           sin θ =
    I                                      S
                                               E
                                                           hypotenuse
    T
    E                                      θ
Using sine to find missing lengths

      S O H C A H TO A                             Sin   θ   =
                                                                  opposite
                                                                 hypotenuse

                                         opp
            hyp              Sin θ   =
                                         hyp
        11 cm          65°                       Sin 65 x 11 = x
                                         x
                             Sin 65 =              9.97 (2dp) = x
                                         11
           x cm
          opposite


                   x                        x                                 x
        Sin 32 =                   Sin 72 =                        Sin 54 =
                   6                        12                                15
      Sin 32 x 6 = x          Sin 72 x 12 = x                    Sin 54 x 15 = x
      3.18cm (2dp) = x         11.41cm (2dp) = x                 12.14cm (2dp) = x
          6 cm         32°           12 cm       72°                   15 cm      54°
(1)                          (2)                             (3)

                                         x cm                             x cm
Using sine to find missing lengths

  S O H C A H TO A                              Sin   θ       =
                                                                  opposite
                                                                  hypotenuse

                                     opp
          hyp            Sin θ   =
                                     hyp
      x cm         47°                                    8
                                     8         x =
                         Sin 47 =                    Sin 47
                                     x
         8 cm opposite                         x = 10.94 cm (2dp)
          6
 Sin 32 = x                        12                                   3
                          Sin 72 = x                          Sin 54 = x
  x = 6                            12
       Sin 32              x =                                 x = 3
                                 Sin 72                              Sin 54
  x = 11.32 cm(2dp)
                           x = 12.62 cm(2dp)                  x = 3.71 cm(2dp)
        x cm       32°           x cm         72°                  x cm        54°
      hyp
(1)                       (2)                                  (3)

             6 cm opp                 12 cm                               3 cm
Using cosine to find missing lengths
                                                                adjacent
                                                     cos θ =
 S O H C A H TO A                                              hypotenuse

                                   adj
                       cos θ   =                           cos θ     =
                                                                         adj
                                   hyp                                   hyp
        hyp                                    hyp
                       cos 53 = x                                        12
  10 cm                         10       x cm                  cos 66  = x

       53°         cos 53 x 10 = x                               x = 12
                                               66°                    cos 66
        x cm         6.02 (2dp) = x             12 cm
           adj                                                  x = 29.50 cm(2dp)
                                                   adj

                                 adj
                     cos θ     = hyp                      cos θ
                                                                      adj
                                                                    = hyp
        hyp                              (2)
(1)                   cos 31 = x                                    22
      9 cm                     9         x cm              cos 49 = x

        31°       cos 31 x 9 = x           49°               x = 22
                                                                 cos 49
         x cm       7.71 (2dp) = x          22 cm
                                                               x = 33.53 cm(2dp)
            adj
Using tangent to find missing lengths
                                                                       opposite
 S O H C A H TO A                                      tan   θ     =
                                                                       adjacent

                                                             opp
                                       opp
                           tan θ                             8cm               xcm
                                   =                                             adj
                                       adj                                31
     71°
 10 cm
adj                        tan 71 = x
                                    10                                    opp
                                                             tan θ      = adj
        opp x cm     tan 71 x 10 = x
                                                                       8
                     29.04cm (2dp)= x                         tan 31 = x

                                                                x = 8
                                                                    tan 31
            7c
              m            (2)           43                      x = 13.31 cm(2dp)
(1)                                         c   m
  m




               31
xc




        4.21cm (2dp) = x
                                                48           x = 38.72 cm(2dp)
                                                     xc
                                                       m
Using all 3 trig ratios to find missing lengths

S O H C A H TO A                                         hyp
                                                      10 cm

      opp                              opp
                                                        53°
      7cm                           7cm                  x cm
                     47                                     adj
                           33°
                                                                    adj
          xcm hyp           x cm adj                   cos θ   =
                                                                    hyp

               opp                  opp                cos 53 = x
sin   θ    =              tan θ   = adj                         10
               hyp
          7                         7              cos 53 x 10 = x
 sin 47 = x                tan 33 = x
                                                6.02cm (2dp)       = x
   x = 7                     x = 7
       sin 47                    tan 33
  x = 9.57 cm(2dp)          x = 10.78 cm(2dp)
Finding Angles using Trig

S O H C A H TO A                                    6cm
                                                          opp
                                    θ
            8 cm opp                hyp 10cm


                                                  opp
 5 cm
                 tan θ =
                          opp        sin   θ    =
adj                                               hyp
        θ                 adj
                                                 6
                tan θ =
                         8          sin    θ   =
                                                 10
                         5
                                                 6
                                           θ   =    sin-1
                  θ = 8 tan-1
                       5
                                                 10

                      57.99 (2dp)         θ    = 36.87 (2dp)
Friday 10 August 2012
             Trigonometry 2
   Objective: Use trig to find missing lengths
    and angles in right angled triangles for
          worded questions. Grade A

Outcomes

Must: Use trig ratios to find missing lengths and
angles in triangles.

Should: Use trig to answer worded problems.

Could: Use trig to answer more difficult worded
problems.
Finding side lengths
A 5 m long ladder is resting against a wall. It makes an
angle of 70° with the ground.
                         What is the distance between the
                         base of the ladder and the wall?

                  We are given the hypotenuse and we want
                  to find the length of the side adjacent to
          5m      the angle, so we use:
                                            adjacent
                                cos θ =
                                          hypotenuse
        70°                               x
        x                     cos 70° =
                                          5
                                    x = 5 × cos 70°
                                      = 1.71 m (to 2d.p.)
Friday 10 August 2012

          Area of a triangle
            (using ½ ab Sin C)
                     Outcomes

                     Must: Understand when you
Objective
                     can use this formula for
Find the area of a   triangle area.
triangle using
Area = ½ ab sin C
                     Should: Be able to find the
                     area of a triangle using
                     ½ ab sin C.

                     Could: Answer exam questions
Triangle Area : When to use ½ ab Sin C ?
• When you have not been given a perpendicular height
(straight height).                                      4cm       40
                                                                       6cm
• When you have been given two lengths and an
angle between them.
                         A
                                                        A = ½ ab Sin C

                                  5cm             A = ½ x 4 x 6 Sin 35
                             35
                                                  A = ……….cm2
   B                               C
                   4cm
                         1                               6cm
  Area of triangle ABC =   ab sin C
                         2                                 23
                                                                9cm
               A = ½ x 4 x 5 Sin 35                   A = ½ ab Sin C
               A = ……….cm2                         A = ½ x 6 x 9 Sin 35
                                                   A = ……….cm2
Find the area of the triangles


(1)                         (2)
      6cm           4cm           6cm            4cm

                  50              35
                                                                 (3)
            7cm                          7cm
                                                              5cm        45
A = ½ ab Sin C             A = ½ ab Sin C                                     3cm
A=    ½   x 7 x 4 Sin 50   A=     ½    x 7 x 4 Sin 35
A=    ……….cm2              A=     ……….cm2
                                                                    7cm

                                                        A = ½ ab Sin C
                                                        A=   ½   x 5 x 3 Sin 45
                                                        A=   ……….cm2
Friday 10 August 2012


                        Sine Rule

                              Outcomes
Objective
Use sine rule to find
angles and lengths in         Must: Use sine rule to find
triangles. (not right         lengths in triangles.
angles triangles)
                              Should: Use sine rule to find
                              angles in triangles.

                              Could: Answer mixed
                              questions
Sine Rule                                   Examples: Find y using
                                            sine rule
• Find the sides and angles of a triangle
whether it’s a right angle or not.
                                                   b 6cm
                       C                                                     y cm
                                                                                   a
                                                       25               40
                                            A                                  B
              b             a                         a           b
                                                            =
                                                   Sin A        Sin B
    A                           B                           y
                  c                                 3
                                                        =
                                                  Sin 25 Sin 40
      a       b       c
          =       =                               3
    sin A   sin B   sin C                                 x Sin 40 = y
                                                Sin 25

                                                      4.56 cm (2dp) = y
Sine Rule                                 a         b             c
                              C                 =           =
                                        Sin A       Sin B       Sin C
                   b                a
         A                c             B                                            (3)                C
Exercise: Find lengths y using sine rule                                                        b
                                                                                           8.9cm
                                                     (2)                    C
  (1)                           C
                                                                b
                              34                           4.2cm        41                 36               62
                                                                                                                 B
                                         y cm                                                       y   c
                                            a
                 15                                                             22
                                                                                     B
 A                                                                  y       c
                          6cm c
                                                                                                    Y = 5.9 cm
     y                6                                4.2              y
             =                                             =
Sin 15           Sin 34                              Sin 22 Sin 41
                      6                               4.2
     y =                      x Sin 15                          x Sin 41 = y
                 Sin 34                              Sin 22
     y = 2.8 cm (1dp)                                       7.4 cm (1dp) = y
Sine Rule
                                                       C
                                   b
                   C
                                   2.3cm             43

          b            a                                   x
                                          3.5cm                B
 A                         B               c
              c
                                  Sin B              Sin C
 Use this when finding a length              =
                                   b                  c
   a       b       c                                  Sin 43
       =       =                   Sin x
 sin A   sin B   sin C                           =
                                       2.3             3.5
                                                      Sin 43
              or                   Sin x =                     x 2.3
                                                       3.5
Use this when finding an angle
  sin A   sin B   sin C            Sin x = 0.44817…..
        =       =                        x = 0.44817…..sin-1
    a       b       c
                                         x = 26.6 (1dp)
Exercise: Find angle y using sin rule
Sine Rule
                                                        C                            (2)         C
                                (1)
                                                        63                                       y
                C                                              2.9 cm           7.3 cm
                                                                                b
                                                                a
        b           a                     y                                                          53
                            A                                                                             B
A           c           B                        4.3cm c                                   8.4cm c
Sin A
        = Sin B = Sin C      Sin A
                                          =
                                               Sin C                    Sin B
                                                                                 =
                                                                                        Sin Y
 a         b       c              a              c                       b                 c
                            Sin y             Sin 63                    Sin 53          Sin y
                                      =                                             =
                             2.9               4.3                       7.3               8.4
                                              Sin 63                    Sin 53
                            Sin y =                    x 2.9                        x 8.4 = Sin y
                                      4.3                                7.3
                            Sin y = 0.600911…..                             0.9080… Sin y
                                                                                   =
                                 y = 0.600911….. -1
                                              sin                       0.9080… sin-1 = y
                                 y = 36.9 (1dp)                                 65.2 (1dp) = y
Sine Rule Practice

Higher GCSE for AQA (Oxford) Book

Page 387 Exercise 4r Q1 – Q19
Friday 10 August 2012


                    Cosine Rule

                           Outcomes
Objective
Use cosine rule to find
angles and lengths in      Must: Use cosine rule to find
triangles. (not right      lengths in triangles.
angles triangles)
                           Should: Use cosine rule to
                           find angles in triangles.

                           Could: Answer mixed
                           questions
The cosine rule                                              Use when
                                                             finding a length
                                  a2 = b2 + c2 – 2bc cos A   of a side.
                A
                                           or
       c            b                                        Use when
                                cos A =     b2 + c2 – a2     finding an
B                       C                       2bc          Angle.
            a


                        When you are given two lengths and an angle
Example 1               between them use;
Find a          B
                                    a2 = b2 + c2 – 2bc cos A
        a                4 cm       a2 = 72 + 42 - 2 x 7 x 4 cos 48
                                    a2 = 27.52868…..
C                       48°   A     a = 5.25 (2dp)
            7 cm
When you are given two lengths and an angle
   Example 1              between them use;
   Find a            B
                                     a2 = b2 + c2 – 2bc cos A
              a            4 cm      a2 = 72 + 42 - 2 x 7 x 4 cos 48
                                     a2 = 27.52868…..
   C                      48°   A    a = 5.25 (2dp)
                   7 cm
Exercise Find the length marked x

 (1)                                  (2)
             2cm
                                                  x          9cm
       52
                                                       98
                     x
            5cm                                 5cm

                                            a2 = 52 + 92 - 2 x 5 x 9 cos 98
a2 = 52 + 22 - 2 x 5 x 2 cos 52
                                            a2 = 118.52557…..
a2 = 16.68677…..
                                            a = 10.89 (2dp)
a = 4.08 (2dp)
The cosine rule                                                    Use when
                                                                   finding a length
                                   a2 = b2 + c2 – 2bc cos A        of a side.
                   A                                               (given 2 lengths
                                             or                    and an angle)
         c                 b
                                   cos A =    b2 + c2 – a2         Use when
 B                             C                  2bc              finding an
               a                                                   Angle.
                                                               (given 3 lengths)

Example 1      You are given 3 sides and asked for an angle.
Find angle A
                       B
                                    cos A = b + c - a
                                             2   2    2

          a                                    2bc
     8 cm              c
                       6 cm         cos A = 4 + 6 - 8
                                               2    2 2


                                              2x4x6
 C     4 cm        A                 cos A = - 0.25
     b
                                         A = - 0.25 cos-1        A = 104. 5 (1dp)
Example 1        You are given 3 sides and asked for an angle.
 Find angle A
                              cos A = b + c - a
                                       2   2   2
                        B
                                         2bc
            a
        8 cm            c  cos A = 4 + 6 - 8
                                      2    2    2

                      6 cm
                                     2x4x6
                                                              A= 104. 5 (1dp)
  C                 A       cos A = - 0.25
       b
           4 cm
                                A = - 0.25 cos-1
Exercise Find the length marked x
                                          (2)             a
                                                       11cm             b
 (1)         b                                                        9cm
            2cm
                                                              x
 A     x                 a                                        A
                        4cm                        c 5cm
        c 5cm                             cos A = 9 + 5 - 11
                                                   2   2     2

                                                   2x9x5
cos A = 2 + 5 - 4
           2  2   2


          2x2x5                            cos A = - 0.1666….
 cos A = 0.65                                   A = - 0.166..cos-1 A = 99. 6 (1dp)
       A = 0.65 cos-1       A = 49.5 (1dp)

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Cosine and sine_rule

  • 1. Friday 10 August 2012 Trigonometry Using all 3 trig ratios to find missing lengths Outcomes Must: Use label sides of triangles correctly Should: Be able to do calculations involving trig functions Could: Use trig ratios to find missing lengths in triangles.
  • 2. Friday 10 August 2012 Right-angled triangles A right-angled triangle contains a right angle. The longest side opposite the right angle is called the hypotenuse.
  • 3. The opposite and adjacent sides The two shorter sides of a right-angled triangle are named with respect to one of the acute angles. The side opposite the marked angle is called the opposite side. The side between the x marked angle and the right angle is called the adjacent side.
  • 5. Calculate the following ratios Use your calculator to find the following to 3 significant figures. 1) sin 79° = 0.982 2) cos 28° = 0.883 3) tan 65° = 2.14 4) cos 11° = 0.982 5) sin 34° = 0.559 6) tan 84° = 9.51 7) tan 49° = 1.15 8) sin 62° = 0.883 9) tan 6° = 0.105 10) cos 56° = 0.559
  • 6. The three trigonometric ratios Opposite O H Sin θ = Hypotenuse SOH Y P P P O O T Adjacent S I E N Cos θ = Hypotenuse CAH T U E S E θ Opposite Tan θ = Adjacent TOA ADJACENT Remember: S O H C A H T O A
  • 7. The sine ratio the length of the opposite side The ratio of is the sine ratio. the length of the hypotenuse The value of the sine ratio depends on the size of the angles in the triangle. O H P Y P We say: P O T O E opposite S N U sin θ = I S E hypotenuse T E θ
  • 8. Using sine to find missing lengths S O H C A H TO A Sin θ = opposite hypotenuse opp hyp Sin θ = hyp 11 cm 65° Sin 65 x 11 = x x Sin 65 = 9.97 (2dp) = x 11 x cm opposite x x x Sin 32 = Sin 72 = Sin 54 = 6 12 15 Sin 32 x 6 = x Sin 72 x 12 = x Sin 54 x 15 = x 3.18cm (2dp) = x 11.41cm (2dp) = x 12.14cm (2dp) = x 6 cm 32° 12 cm 72° 15 cm 54° (1) (2) (3) x cm x cm
  • 9. Using sine to find missing lengths S O H C A H TO A Sin θ = opposite hypotenuse opp hyp Sin θ = hyp x cm 47° 8 8 x = Sin 47 = Sin 47 x 8 cm opposite x = 10.94 cm (2dp) 6 Sin 32 = x 12 3 Sin 72 = x Sin 54 = x x = 6 12 Sin 32 x = x = 3 Sin 72 Sin 54 x = 11.32 cm(2dp) x = 12.62 cm(2dp) x = 3.71 cm(2dp) x cm 32° x cm 72° x cm 54° hyp (1) (2) (3) 6 cm opp 12 cm 3 cm
  • 10. Using cosine to find missing lengths adjacent cos θ = S O H C A H TO A hypotenuse adj cos θ = cos θ = adj hyp hyp hyp hyp cos 53 = x 12 10 cm 10 x cm cos 66 = x 53° cos 53 x 10 = x x = 12 66° cos 66 x cm 6.02 (2dp) = x 12 cm adj x = 29.50 cm(2dp) adj adj cos θ = hyp cos θ adj = hyp hyp (2) (1) cos 31 = x 22 9 cm 9 x cm cos 49 = x 31° cos 31 x 9 = x 49° x = 22 cos 49 x cm 7.71 (2dp) = x 22 cm x = 33.53 cm(2dp) adj
  • 11. Using tangent to find missing lengths opposite S O H C A H TO A tan θ = adjacent opp opp tan θ 8cm xcm = adj adj 31 71° 10 cm adj tan 71 = x 10 opp tan θ = adj opp x cm tan 71 x 10 = x 8 29.04cm (2dp)= x tan 31 = x x = 8 tan 31 7c m (2) 43 x = 13.31 cm(2dp) (1) c m m 31 xc 4.21cm (2dp) = x 48 x = 38.72 cm(2dp) xc m
  • 12. Using all 3 trig ratios to find missing lengths S O H C A H TO A hyp 10 cm opp opp 53° 7cm 7cm x cm 47 adj 33° adj xcm hyp x cm adj cos θ = hyp opp opp cos 53 = x sin θ = tan θ = adj 10 hyp 7 7 cos 53 x 10 = x sin 47 = x tan 33 = x 6.02cm (2dp) = x x = 7 x = 7 sin 47 tan 33 x = 9.57 cm(2dp) x = 10.78 cm(2dp)
  • 13. Finding Angles using Trig S O H C A H TO A 6cm opp θ 8 cm opp hyp 10cm opp 5 cm tan θ = opp sin θ = adj hyp θ adj 6 tan θ = 8 sin θ = 10 5 6 θ = sin-1 θ = 8 tan-1 5 10 57.99 (2dp) θ = 36.87 (2dp)
  • 14.
  • 15. Friday 10 August 2012 Trigonometry 2 Objective: Use trig to find missing lengths and angles in right angled triangles for worded questions. Grade A Outcomes Must: Use trig ratios to find missing lengths and angles in triangles. Should: Use trig to answer worded problems. Could: Use trig to answer more difficult worded problems.
  • 16. Finding side lengths A 5 m long ladder is resting against a wall. It makes an angle of 70° with the ground. What is the distance between the base of the ladder and the wall? We are given the hypotenuse and we want to find the length of the side adjacent to 5m the angle, so we use: adjacent cos θ = hypotenuse 70° x x cos 70° = 5 x = 5 × cos 70° = 1.71 m (to 2d.p.)
  • 17.
  • 18. Friday 10 August 2012 Area of a triangle (using ½ ab Sin C) Outcomes Must: Understand when you Objective can use this formula for Find the area of a triangle area. triangle using Area = ½ ab sin C Should: Be able to find the area of a triangle using ½ ab sin C. Could: Answer exam questions
  • 19. Triangle Area : When to use ½ ab Sin C ? • When you have not been given a perpendicular height (straight height). 4cm 40 6cm • When you have been given two lengths and an angle between them. A A = ½ ab Sin C 5cm A = ½ x 4 x 6 Sin 35 35 A = ……….cm2 B C 4cm 1 6cm Area of triangle ABC = ab sin C 2 23 9cm A = ½ x 4 x 5 Sin 35 A = ½ ab Sin C A = ……….cm2 A = ½ x 6 x 9 Sin 35 A = ……….cm2
  • 20. Find the area of the triangles (1) (2) 6cm 4cm 6cm 4cm 50 35 (3) 7cm 7cm 5cm 45 A = ½ ab Sin C A = ½ ab Sin C 3cm A= ½ x 7 x 4 Sin 50 A= ½ x 7 x 4 Sin 35 A= ……….cm2 A= ……….cm2 7cm A = ½ ab Sin C A= ½ x 5 x 3 Sin 45 A= ……….cm2
  • 21.
  • 22. Friday 10 August 2012 Sine Rule Outcomes Objective Use sine rule to find angles and lengths in Must: Use sine rule to find triangles. (not right lengths in triangles. angles triangles) Should: Use sine rule to find angles in triangles. Could: Answer mixed questions
  • 23. Sine Rule Examples: Find y using sine rule • Find the sides and angles of a triangle whether it’s a right angle or not. b 6cm C y cm a 25 40 A B b a a b = Sin A Sin B A B y c 3 = Sin 25 Sin 40 a b c = = 3 sin A sin B sin C x Sin 40 = y Sin 25 4.56 cm (2dp) = y
  • 24. Sine Rule a b c C = = Sin A Sin B Sin C b a A c B (3) C Exercise: Find lengths y using sine rule b 8.9cm (2) C (1) C b 34 4.2cm 41 36 62 B y cm y c a 15 22 B A y c 6cm c Y = 5.9 cm y 6 4.2 y = = Sin 15 Sin 34 Sin 22 Sin 41 6 4.2 y = x Sin 15 x Sin 41 = y Sin 34 Sin 22 y = 2.8 cm (1dp) 7.4 cm (1dp) = y
  • 25. Sine Rule C b C 2.3cm 43 b a x 3.5cm B A B c c Sin B Sin C Use this when finding a length = b c a b c Sin 43 = = Sin x sin A sin B sin C = 2.3 3.5 Sin 43 or Sin x = x 2.3 3.5 Use this when finding an angle sin A sin B sin C Sin x = 0.44817….. = = x = 0.44817…..sin-1 a b c x = 26.6 (1dp)
  • 26. Exercise: Find angle y using sin rule Sine Rule C (2) C (1) 63 y C 2.9 cm 7.3 cm b a b a y 53 A B A c B 4.3cm c 8.4cm c Sin A = Sin B = Sin C Sin A = Sin C Sin B = Sin Y a b c a c b c Sin y Sin 63 Sin 53 Sin y = = 2.9 4.3 7.3 8.4 Sin 63 Sin 53 Sin y = x 2.9 x 8.4 = Sin y 4.3 7.3 Sin y = 0.600911….. 0.9080… Sin y = y = 0.600911….. -1 sin 0.9080… sin-1 = y y = 36.9 (1dp) 65.2 (1dp) = y
  • 27.
  • 28.
  • 29.
  • 30. Sine Rule Practice Higher GCSE for AQA (Oxford) Book Page 387 Exercise 4r Q1 – Q19
  • 31. Friday 10 August 2012 Cosine Rule Outcomes Objective Use cosine rule to find angles and lengths in Must: Use cosine rule to find triangles. (not right lengths in triangles. angles triangles) Should: Use cosine rule to find angles in triangles. Could: Answer mixed questions
  • 32. The cosine rule Use when finding a length a2 = b2 + c2 – 2bc cos A of a side. A or c b Use when cos A = b2 + c2 – a2 finding an B C 2bc Angle. a When you are given two lengths and an angle Example 1 between them use; Find a B a2 = b2 + c2 – 2bc cos A a 4 cm a2 = 72 + 42 - 2 x 7 x 4 cos 48 a2 = 27.52868….. C 48° A a = 5.25 (2dp) 7 cm
  • 33. When you are given two lengths and an angle Example 1 between them use; Find a B a2 = b2 + c2 – 2bc cos A a 4 cm a2 = 72 + 42 - 2 x 7 x 4 cos 48 a2 = 27.52868….. C 48° A a = 5.25 (2dp) 7 cm Exercise Find the length marked x (1) (2) 2cm x 9cm 52 98 x 5cm 5cm a2 = 52 + 92 - 2 x 5 x 9 cos 98 a2 = 52 + 22 - 2 x 5 x 2 cos 52 a2 = 118.52557….. a2 = 16.68677….. a = 10.89 (2dp) a = 4.08 (2dp)
  • 34. The cosine rule Use when finding a length a2 = b2 + c2 – 2bc cos A of a side. A (given 2 lengths or and an angle) c b cos A = b2 + c2 – a2 Use when B C 2bc finding an a Angle. (given 3 lengths) Example 1 You are given 3 sides and asked for an angle. Find angle A B cos A = b + c - a 2 2 2 a 2bc 8 cm c 6 cm cos A = 4 + 6 - 8 2 2 2 2x4x6 C 4 cm A cos A = - 0.25 b A = - 0.25 cos-1 A = 104. 5 (1dp)
  • 35. Example 1 You are given 3 sides and asked for an angle. Find angle A cos A = b + c - a 2 2 2 B 2bc a 8 cm c cos A = 4 + 6 - 8 2 2 2 6 cm 2x4x6 A= 104. 5 (1dp) C A cos A = - 0.25 b 4 cm A = - 0.25 cos-1 Exercise Find the length marked x (2) a 11cm b (1) b 9cm 2cm x A x a A 4cm c 5cm c 5cm cos A = 9 + 5 - 11 2 2 2 2x9x5 cos A = 2 + 5 - 4 2 2 2 2x2x5 cos A = - 0.1666…. cos A = 0.65 A = - 0.166..cos-1 A = 99. 6 (1dp) A = 0.65 cos-1 A = 49.5 (1dp)

Editor's Notes

  1. Review the definition of a right-angled triangle. Tell pupils that in any triangle the angle opposite the longest side will always be the largest angle and vice-versa. Ask pupils to explain why no other angle in a right-angled triangle can be larger than or equal to the right angle. Ask pupils to tell you the sum of the two smaller angles in a right-angled triangle. Recall that the sum of the angles in a triangle is always equal to 180 °. Recall, also, that two angles that add up to 90° are called complementary angles. Conclude that the two smaller angles in a right-angled triangle are complementary angles.
  2. We don ’t don’t need to know the actual size of the marked angle to label the two shorter sides as shown. It could be labelled using a letter symbol such as x as shown here. Point out that if we labelled the sides with respect to the other acute angle their name would be reversed.
  3. Use this activity to practice labelling the sides of a right-angled triangle with respect to angle θ . Ask volunteers to come to the board to complete the activity.
  4. This exercise practices the use of the sin, cos and tan keys on the calculator. It also practices rounding to a given number of significant figures. Pupils should notice that the sine of a given angle is equal to the cosine of the complement of that angle. They can use this fact to answer question 10 using the answer to question 5.
  5. Stress to pupils that they must learn these three trigonometric ratios. Pupils can remember these using SOHCAHTOA or they may wish to make up their own mnemonics using these letters.
  6. The sine ratio depends on the size of the opposite angle. We say that the sine of the angle is equal to the length of the opposite side divided by the length of the hypotenuse. Sin is mathematical shorthand for sine. It is still pronounced as ‘sine’.
  7. This ratio can also be demonstrated using the similar right-angled activity on slide 7.
  8. This ratio can also be demonstrated using the similar right-angled activity on slide 7.
  9. This ratio can also be demonstrated using the similar right-angled activity on slide 7.
  10. This ratio can also be demonstrated using the similar right-angled activity on slide 7.
  11. We can use the first form of the formula to find side lengths and the second form of the equation to find angles.
  12. We can use the first form of the formula to find side lengths and the second form of the equation to find angles.