2000 years ago triangles were discovered and then came the Pythagorean Theorem. Basically, it means that in a right triangle the hypotenuse (the longest side of the triangle)  can be found by this equation. 

a2 + b2 = c2

The letters represent numbers and 2 represents square. You have to square the numbers and add them and then square root the sum. This will give you the answer.

example:

a2 + b2 = c2

52 + 122 = c2
25 + 144 = c2
25+144=169
169 = c2
c2 = 169
c = √169
c = 13