Find an unknown side of a right-angled triangle by Pythagoras’ theorem

Follow this one with me, a step at a time. Try to guess each step first.

Worked example 1

In △ABC, ∠A = 90°, AB = 20 cm and AC = 48 cm. Find BC in cm.

Remember: The right angle is at A, so the hypotenuse is the side opposite A. Which side is that?

BC is opposite the right angle, so it is the hypotenuse: AB² + AC² = BC².
BC² = 20² + 48², and 400 + 2304 = 2704.
BC is the square root of 2704, so BC = 52 cm.

Common mistakes

Subtracted the lengths. Pythagoras’ theorem works with their squares.
Added the lengths. Pythagoras’ theorem adds their squares.
Stopped at the square of the length. Take the square root at the end.