Why can the Pythagorean Theorem only be used with Right Triangles?
It's not really true. The Pythagorean Theorem (its converse, really) can be used on any triangle to tell us whether or not it is a right triangle.
For example, let's check the triangle with sides 2,3,4:
But of course
The Pythagorean Theorem is a special case of the Law Of Cosines for