The Pythagorean theorem is the idea that the sum of the two legs which are both squared is equal to the hypotenuse's length squared.
*look at the image I attached for a better explanation
By looking at the picture, we are missing the leg's length, and by using the Pythagorean theorem, we get the equation
[tex]?^2+5^2 = 13^2\\?^2 + 25 = 169\\?^2 = 144\\? = 12[/tex]
Thus the missing side length is 12 cm
Hope that helps!
