For a triangle with sides $a$ and $b,$ and $c,$ if the equality $a^2+b^2=c^2$ holds true, the triangle is a right triangle.

The equality holds true for the given triangle, since $3^2+4^2=5^2.$ The theorem then states that it is a right triangle. The right angle is opposite side $c.$ The theorem is named the Converse Pythagorean Theorem because the same thing holds true but in opposite order.