The pythagorean theorem states: a² + b² = c²
Where c is the longest side (hypotenuse).
So we plug in and solve for the third side "b".
a = 12, c = 13, so:
12² + b² = 13²
144 + b² = 169
b² = 169 - 144
b² = 25
b = 5
We can double check the pythagorean theorem to make sure:
12² + 5² = 13²
144 + 25 = 169
So the unknown side is definitely 5.