Pythagoras' Theorem

Pythagoras was a Greek philosopher who discovered that in a right angled triangle (triangle with one angle 90 degrees) the hypotenuse squared , c², was equal to the sum of the other sides squared, a and b. The hypotenuse is the longest edge and is opposite the right angle, a and b are the adjacent sides, next too, the right angle. 

We call this Pythagoras' Theorem: a²+b²=c²

If we know two of the three sides we can use this theorem to work out the other, by subbing in the values known and rearranging.