How do you find a missing side length in a right-angled triangle?

Use Pythagoras' theorem. a² + b² = c². The longest side of the triangle is the hypotenuse, and it is the side opposite the right angle. We use the length of this side as 'c' if we have it. 'a' and 'b' are the other two sides, and it doesn't matter which is which.We can rearrange the above equation as c² - b² = a² OR c² - a² = b². This allows us to find the missing side using the equation above if we need to find the hypotenuse, or one of these other two equations if looking for a shorter side. Remember to square root at the end to find the side length.