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+9^2 = 16^2\\?^2 + 81=256\\?^2 = 175\\? = 13.2[/tex]
Thus the missing side length is 13.2 cm
Hope that helps!
