Pythagoras’ theorem states that for a right-angled triangle:
The square of the hypotenuse of a right-angled triangle equals the sum of the squares of the lengths of the other two sides.
In symbols, $$c^2=a^2+b^2$$.
The converse
If $$c^2=a^2+b^2$$ in a triangle ABC, then ∠C is a right angle.