ForMuLas FoR GeoMeTry

Formulas for
Geometry
Mr. Ryan

Visit www.worldofteaching.com for more free powerpoints

Don’t Get Scared!!!
•Evil mathematicians have created
formulas to save you time. But, they
always change the letters of the
formulas to scare you!

2
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Perimeter
•Any shape’s “perimeter” is the
outside of the shape…like a fence
around a yard.

3

Perimeter
•To calculate the perimeter of any
shape, just add up “each” line
segment of the “fence”.

4

Perimeter
•Triangles have 3 sides…add up each
sides length.

8
8

8+8+8=24
8 The Perimeter is 24
5

Perimeter
•A square has 4 sides of a fence
12

12 12

12+12+12+12=48
12
6

Perimeter
•Sometimes, problems may only give
you two measurements for a square
or rectangle.
•No problem…use the formula for
squares/rectangles (only!)

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Remember
• Squares ALL sides are equal…so if they give you
one side, you know ALL the sides
• Length=the Largest side
• If they “leave” numbers out, they are equal to
their opposite side. If they give you the bottom of a
square/rectangle type shape then the top is the
same

8

The Same!!
•If the bottom is 15…the top is…

15
9

Square/Rectangle Formula
• Perimeter=2(Length+Width)
• P= 2(25+14) P=2(20+20)
• P= 50+28 P=40+40
• P= 78 P=80

14

20
25
10

Other shapes
•Just add up EACH segment

10

8 sides, each side 10 so 10+10+10+10+10+10+10+10=80

11

Odd shapes
• Count ALL sides
• Remember if one side blank, it’s equal to its opposite
25+25=50 (for Length) 15+5+15+5=40 (for Width)

5

15

Perimeter=90
25
12

Area
•Area is the ENTIRE INSIDE of a
shape
•It is always measured in “squares”
(sq. inch, sq ft)

13

AREA
• Square units means that “that” many squares fit inside that shape (if measured
in feet…it’s feet…if meters…it’s meters. In this example the area is 4 square
units…note 4 squares fit)

1 2

3 4

2 units (ft, in, m)
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Area of Squares/Rectangles
•Length x Width=Area

2

Length(2) xWidth(2) = 4 square units
15

Parallelogram Area
•Same idea as squares & rectangles,
but they change the words to Base
(length:bottom of shape) and height
(width)

16

Parallelogram Area
•Area=Base x Height
•(Area=length x width)
The diagonal line is NOT
5 the height!!!
Height (width)

8
BASE (length)
Base 8 x Height 5 = Area 40
17

REAL formula for area of
squares
•Area= S^2
•Area= Side x Side (side squared)
(just a different way of saying
length x width)
Side one

Side two
18

Different Names/Same idea
•Length x Width = Area

•Side x Side = Area

•Base x Height = Area

19

Area of a Triangle
•½ Base x Height = Area
•(It’s ½ because ½ of the “square” is
missing) Height
5
Base
8

Height
½ Base x Height = Area
½ (8) x 5 = Area
4 x 5 = 20
Base
20

Area of a Circle
•Pi= 3.14
•Radius: from center (origin) of
circle to ANY side
•Area= pi x (Radius x Radius)

21

Circumference of a Circle
• Perimeter/Circumference = pi x diameter
• Pi is always 3.14
• Circumference is a fancy name for perimeter
• The diameter is a line from one side to the other
side of a circle through its origin (It’s twice the
radius)

If the radius is 5, then the diameter is 10

22

Area of a circle
•Area = piR^2
•Pi = 3.14 and R=Radius

5
Radius

Area=3.14 x (5 x 5) Perimeter = 3.14 x 10

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You’re Done
•Squares, rectangles and parallelograms
area are almost the same (LxW)
•Triangles are ½ cause your missing ½
•Circles have fancy names, but just follow
the formula

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
•For most tests you will have the
formulas given to you. Just
remember which one to use for which
shape and you’ll do fine.

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