The Pythagorean Theorem also states that the sum of the squares of the lengths of the legs of a right triangle is equal to the square of the length of the hypotenuse.
In other words, for the triangle shown below, a² + b² = c².
In the image below, I have drawn a triangle on my whiteboard.
In the triangle that is shown below, the legs, a and b, are the perpendicular sides of the right triangle and the hypotenuse, c, is the side opposite the right angle.
This only applies to right triangles!
You can use this theorem to find out if a triangle is an acute triangle, a right triangle, or even an obtuse triangle.