What's Pythagoras Theorem and how can I use it?

Pythagoras theorem is: A^2 + B^2 = C^2. This might look confusing but it is actually quite simple. Pythagoras can only be used on a RIGHT ANGLED TRIANGLE! Then what you do is that the side of the triangle OPPOSITE to the RIGHT ANGLE is the hypotenuse, and you label this C (from the equation) the other two sides can be either A or B depending on what you fancy, but the hypotenuse must always by C. The simply square the sides and the equation will work. You need to have 2 of the sides of the triangle to work out the third.