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Module 9: Geometry

Perimeter

Perimeter is the total length of the outer edges of a shape. This can be obtained by measuring the length of the individual sides of a shape, and then adding them together. This is particularly useful when dealing with shapes that don’t have identical sides. However, there are some shortcuts and formulas that can be used for shapes that have identical sides. Some examples are included in the tables below.

In these tables, P {"version":"1.1","math":"P"} = perimeter, l {"version":"1.1","math":"l"} = length, w {"version":"1.1","math":"wl"} = width, h {"version":"1.1","math":"h"} = height.

2-Dimensional (2D) Perimeters

Shape Shortcuts and Equations
Equilateral Triange P = 3 l {"version":"1.1","math":"P = 3l"}
Square P = 4 l {"version":"1.1","math":"P = 4l"}
Rectangle/Parallelogram

(with 2 sets of identical sides)

 P = 2 l + 2 w {"version":"1.1","math":"P=2l+2w"}
Circle

The circle is a unique case as the perimeter is known as the circumference ( C {"version":"1.1","math":"C"}). To find it, we’ll need to know the radius ( r {"version":"1.1","math":"r"}) (distance from the centre of the circle to the edge) or the diameter ( d {"version":"1.1","math":"d"}) (distance from one edge to the opposite edge).

C = 2 π r {"version":"1.1","math":"C=2\pi r"}

C = π d {"version":"1.1","math":"C=\pi d"}

Notice: d = 2 r {"version":"1.1","math":"d=2r"}

3-Dimensional (3D) Perimeters

Shapes Shortcuts and Equations
Cube P = 12 l {"version":"1.1","math":"P=12l"}
Rectangular Prism P = 4 l + 4 w + 4 h {"version":"1.1","math":"P=4l+4w+4h"}

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