# How do you find the missing side of a right triangle?

Category: 2D Shapes »

Course: Geometry »

Relevant MM Lessons:
• Forthcoming

## The Third Side

In a right triangle, knowing the lengths of two sides gives us enough information to find the length of the third side, thanks to our trusty friend the Pythagorean Theorem. Famously, the relationship between the lengths of the two legs of a right triangle, $a$ and $b$, and the length of the hypotenuse $c$ is$$a^2 + b^2 = c^2$$When you know two sides, plug them into the formula and solve for the third unknown side. If a leg is missing, let $a$ or $b$ remain unknown and solve for it (it doesn't matter which you choose). If the hypotenuse is missing, let $c$ remain unknown and solve for it.
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