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A
B
C
D
A B C D
Similarity
Similar shapes are always
in proportion to each
other. There is no
distortion between them.
Conditions for similarity
Two shapes are similar only when:
•Corresponding sides are in proportion and
•Corresponding angles are equal
All regular polygons
are similar
Conditions for similarity
Two shapes are similar only when:
•Corresponding sides are in proportion and
•Corresponding angles are equal
All rectangles are not similar to one
another since only condition 2 is true.
If two objects are similar then one is an enlargement of the other
The rectangles below are similar:
Find the scale factor of enlargement that maps A to B
A
B
8 cm
16 cm5 cm
10 cm
Not to scale!
Scale factor = x2
Note that B to A
would be x ½
If two objects are similar then one is an enlargement of the other
The rectangles below are similar:
Find the scale factor of enlargement that maps A to B
A
B
8 cm
12 cm5 cm
7½ cm
Not to scale!
Scale factor = x1½
Note that B to A
would be x 2/3
8 cm
If we are told that two objects are similar and we can find the
scale factor of enlargement then we can calculate the value of an
unknown side.
A
B
C
3 cm
Not to scale!
24 cm
x cm
y cm
13½ cm
The 3 rectangles are similar. Find the
unknown sides
Comparing corresponding sides in A and B: 24/8 = 3 so x = 3 x 3 = 9 cm
Comparing corresponding sides in A and C: 13½ /3 = 4½ so y = 4 ½ x 8 = 36 cm
5.6 cm
If we are told that two objects are similar and we can find the
scale factor of enlargement then we can calculate the value of an
unknown side.
A
B
C
2.1 cm
Not to scale!
x cm
7.14 cm
26.88
cm
y cm
The 3 rectangles are similar. Find the
unknown sides
Comparing corresponding sides in A and B: 7.14/2.1 = 3.4 so x = 3.4 x 5.6 = 19.04 cm
Comparing corresponding sides in A and C: 26.88/5.6 = 4.8 so y = 4.8 x 2.1 = 10.08 cm
Similar Triangles
Similar triangles are important in mathematics and their
application can be used to solve a wide variety of problems.
The two conditions for similarity between shapes as
we have seen earlier are:
•Corresponding sides are in proportion and
•Corresponding angles are equal
Triangles are the exception to this rule.
only the second condition is needed
Two triangles are similar if their
•Corresponding angles are equal
70o 70o
45o
65o
45o
These two triangles are similar since
they are equiangular.
50o
55o
75o
50o
These two triangles are similar since
they are equiangular.
If 2 triangles have 2 angles the
same then they must be equiangular = 180 – 125 = 55
Finding Unknown sides (1)
20 cm
18 cm 12 cm
6 cmb c
Since the triangles are equiangular they are similar.
So comparing corresponding sides.
18
6 12
b
=
6 18
9 cm
12
x
b = =
12
20 18
c
=
20 12
13.3 cm
18
x
c = =
24 cm
10 cm 8 cm
6 cm
x
y
10
6 8
x
=
6 10
7.5 cm
8
x
x = =
8
24 10
y
=
24 8
19.2 cm
10
x
y = =
Comparing corresponding sides
Determining similarity
A B
E D
Triangles ABC and DEC are
similar. Why?C
Angle ACB = angle ECD (Vertically Opposite)
Angle ABC = angle DEC (Alt angles)
Angle BAC = angle EDC (Alt angles)
Since ABC is similar to DEC we know that corresponding
sides are in proportion
AB→DE BC→EC AC→DC
The order of the lettering is important in order to show which
pairs of sides correspond.
A
B C
D E
If BC is parallel to DE, explain why
triangles ABC and ADE are similar
Angle BAC = angle DAE (common to
both triangles)
Angle ABC = angle ADE (corresponding
angles between parallels)
Angle ACB = angle AED (corresponding
angles between parallels)
A
D E
A
D E
B
C
B
C
A line drawn parallel to any side of a triangle produces 2 similar triangles.
Triangles EBC and EAD are similar Triangles DBC and DAE are similar
A
B
C D
E
In the diagram below BE is parallel to CD and all
measurements are as shown.
(a) Calculate the length CD
(b) Calculate the perimeter of the Trapezium EBCD
4.8 m
6 m
4 m
4.5 m
10 m
A
C D8 cm 8 cm
3 cm
7.5 cm
10
( )
4.8 6
CD
a =
4.8 10
8 cm
6
x
CD = =
10
( )
4.5 6
AC
b =
4.5 10
7.5 cm
6
x
AC = =
So perimeter
= 3 + 8 + 4 + 4.8
= 19.8 cm
SF = 10/6 = 1.666..
AC = 1.666… x 4.5 = 7.5
∴Perimeter = 3 + 8 + 4 + 4.8 = 19.8 cm
10 m
A
C D8 cm 8 cm
3 cm
7.5 cm
So BC = 3 cm
So CD = 1.666… x 4.8 = 8 cm
A
B
C D
E
In the diagram below BE is parallel to CD and all
measurements are as shown.
(a) Calculate the length CD
(b) Calculate the perimeter of the Trapezium EBCD
4.8 m
6 m
4 m
4.5 m
Alternative
approach to the
same problem
A
B
C
DE
20 cm
45 cm5 cm
y
The two triangles below are similar: Find the distance y.
5
20 50
y
=
20 5
2 cm
5
x
y = =
Scale Factor = 50/5 = 10 So y = 20/10 = 2 cm
A
B
C
DE
20 cm
45 cm5 cm
y
The two triangles below are similar: Find the distance y.
Alternate
approach to
the same
problem

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7th pre alg similarity & triangles

  • 1. A B C D A B C D Similarity Similar shapes are always in proportion to each other. There is no distortion between them.
  • 2. Conditions for similarity Two shapes are similar only when: •Corresponding sides are in proportion and •Corresponding angles are equal All regular polygons are similar
  • 3.
  • 4. Conditions for similarity Two shapes are similar only when: •Corresponding sides are in proportion and •Corresponding angles are equal All rectangles are not similar to one another since only condition 2 is true.
  • 5. If two objects are similar then one is an enlargement of the other The rectangles below are similar: Find the scale factor of enlargement that maps A to B A B 8 cm 16 cm5 cm 10 cm Not to scale! Scale factor = x2 Note that B to A would be x ½
  • 6. If two objects are similar then one is an enlargement of the other The rectangles below are similar: Find the scale factor of enlargement that maps A to B A B 8 cm 12 cm5 cm 7½ cm Not to scale! Scale factor = x1½ Note that B to A would be x 2/3
  • 7. 8 cm If we are told that two objects are similar and we can find the scale factor of enlargement then we can calculate the value of an unknown side. A B C 3 cm Not to scale! 24 cm x cm y cm 13½ cm The 3 rectangles are similar. Find the unknown sides Comparing corresponding sides in A and B: 24/8 = 3 so x = 3 x 3 = 9 cm Comparing corresponding sides in A and C: 13½ /3 = 4½ so y = 4 ½ x 8 = 36 cm
  • 8. 5.6 cm If we are told that two objects are similar and we can find the scale factor of enlargement then we can calculate the value of an unknown side. A B C 2.1 cm Not to scale! x cm 7.14 cm 26.88 cm y cm The 3 rectangles are similar. Find the unknown sides Comparing corresponding sides in A and B: 7.14/2.1 = 3.4 so x = 3.4 x 5.6 = 19.04 cm Comparing corresponding sides in A and C: 26.88/5.6 = 4.8 so y = 4.8 x 2.1 = 10.08 cm
  • 9. Similar Triangles Similar triangles are important in mathematics and their application can be used to solve a wide variety of problems. The two conditions for similarity between shapes as we have seen earlier are: •Corresponding sides are in proportion and •Corresponding angles are equal Triangles are the exception to this rule. only the second condition is needed Two triangles are similar if their •Corresponding angles are equal
  • 10. 70o 70o 45o 65o 45o These two triangles are similar since they are equiangular. 50o 55o 75o 50o These two triangles are similar since they are equiangular. If 2 triangles have 2 angles the same then they must be equiangular = 180 – 125 = 55
  • 11. Finding Unknown sides (1) 20 cm 18 cm 12 cm 6 cmb c Since the triangles are equiangular they are similar. So comparing corresponding sides. 18 6 12 b = 6 18 9 cm 12 x b = = 12 20 18 c = 20 12 13.3 cm 18 x c = =
  • 12. 24 cm 10 cm 8 cm 6 cm x y 10 6 8 x = 6 10 7.5 cm 8 x x = = 8 24 10 y = 24 8 19.2 cm 10 x y = = Comparing corresponding sides
  • 13. Determining similarity A B E D Triangles ABC and DEC are similar. Why?C Angle ACB = angle ECD (Vertically Opposite) Angle ABC = angle DEC (Alt angles) Angle BAC = angle EDC (Alt angles) Since ABC is similar to DEC we know that corresponding sides are in proportion AB→DE BC→EC AC→DC The order of the lettering is important in order to show which pairs of sides correspond.
  • 14. A B C D E If BC is parallel to DE, explain why triangles ABC and ADE are similar Angle BAC = angle DAE (common to both triangles) Angle ABC = angle ADE (corresponding angles between parallels) Angle ACB = angle AED (corresponding angles between parallels) A D E A D E B C B C A line drawn parallel to any side of a triangle produces 2 similar triangles. Triangles EBC and EAD are similar Triangles DBC and DAE are similar
  • 15. A B C D E In the diagram below BE is parallel to CD and all measurements are as shown. (a) Calculate the length CD (b) Calculate the perimeter of the Trapezium EBCD 4.8 m 6 m 4 m 4.5 m 10 m A C D8 cm 8 cm 3 cm 7.5 cm 10 ( ) 4.8 6 CD a = 4.8 10 8 cm 6 x CD = = 10 ( ) 4.5 6 AC b = 4.5 10 7.5 cm 6 x AC = = So perimeter = 3 + 8 + 4 + 4.8 = 19.8 cm
  • 16. SF = 10/6 = 1.666.. AC = 1.666… x 4.5 = 7.5 ∴Perimeter = 3 + 8 + 4 + 4.8 = 19.8 cm 10 m A C D8 cm 8 cm 3 cm 7.5 cm So BC = 3 cm So CD = 1.666… x 4.8 = 8 cm A B C D E In the diagram below BE is parallel to CD and all measurements are as shown. (a) Calculate the length CD (b) Calculate the perimeter of the Trapezium EBCD 4.8 m 6 m 4 m 4.5 m Alternative approach to the same problem
  • 17. A B C DE 20 cm 45 cm5 cm y The two triangles below are similar: Find the distance y. 5 20 50 y = 20 5 2 cm 5 x y = =
  • 18. Scale Factor = 50/5 = 10 So y = 20/10 = 2 cm A B C DE 20 cm 45 cm5 cm y The two triangles below are similar: Find the distance y. Alternate approach to the same problem