Pythagorean relationship

1. Answer in a short paragraph and with diagrams
Relationship between the sides of the triangle is any right triangle that has one right angle if the length of the legs(A and B) are squared and their sum is found, the sum will equal to the square of the hypotenuse(C).

2. Solve for the missing side length.

3. Is this a right triangle? Prove it!!!

It is not a right triangle because it does not equal 121.