How to Measure Triangles - dummies

By Mark Zegarelli

You can measure the perimeter and area of all triangles. There also is a special feature of right triangles that allows you to measure them more easily.

Finding the perimeter and area of a triangle

Mathematicians have no special formula for finding the perimeter of a triangle — they just add up the lengths of the sides.

To find the area of a triangle, you need to know the length of one side — the base (b for short) — and the height (h). Note that the height forms a right angle with the base. This figure shows a triangle with a base of 5 cm and a height of 2 cm:


Here’s the formula for the area of a triangle:


So here’s how to figure out the area of a triangle with a base of 5 cm and a height of 2 cm:


Finding the third side of a right triangle

The long side of a right triangle (c) is called the hypotenuse, and the two short sides (a and b) are called the legs:


The most important right triangle formula is the Pythagorean theorem:


This formula allows you to find the hypotenuse of a triangle, given only the lengths of the legs. For example, suppose the legs of a triangle are 3 and 4 units. Here’s how to use the Pythagorean theorem to find the length of the hypotenuse:


So when you multiply c by itself, the result is 25. Therefore, take the square root of both sides to find:

c = 5

The length of the hypotenuse is 5 units.