Perimeter is the measure of the distance around a shape. The most basic way to calculate a perimeter is by adding up the length of all the sides. However, there are situations where you won’t be given the length of all the sides, or might just want to calculate the perimeter faster. In these situations you can use formulas to calculate perimeters of specific shapes. These formulas will be available to you during the GED math test.
Perimeter of a Square
Perimeter = 4 x side
When you need to find the perimeter of a square all you need to do is multiply one side by four.
Perimeter of a Rectangle
Perimeter = 2 x length + 2 x width
Multiply one of the short sides by two and multiply one of the long sides by, then add those two numbers together.
Perimeter of a Triangle
Perimeter = side1 + side2 + side3
If the triangle does not have a right angle, the way to find the perimeter is by adding up the length of all three sides.
3 + 4 + 5 = 12
If you need to find the perimeter of a triangle with a right angle, you can find the perimeter even if you only know the lengths of two of the sides.
The way you do that is by multiplying each side by using the Pythagorean Theorem (a² + b² = c ²).
Now you just add the tree sides together as above to get the perimeter.
5 + 4 + 6.4 = 15.4
Circumference of a Circle
Circumference = π x diameter
When you talk about the distance around a circle the term circumference is used instead of radius. The diameter is how wide a circle is at its widest point.
π is approximately 3.14, so just use 3.14 for π (pi).
If you are only given a circle’s radius in a question, you can still find the circumference by multiplying the radius by two before multiplying by π, since the radius is half the diameter.