Pythagorean Theorem
Pythagorean Theorem:
In a right angled triangle, with sides (legs) a and b, and hypotenuse c, then
c² = a²+b².
The square of the hypotenuse is the sum of the squares of the legs.
A "3,4,5" triangle has a right angle in it, so the formula should work.

5^{2}=3^{2}+4^{2}
25=9+16
25=25
Yes, it works!!! 
Apply the Pythagorean Theorem: If you know any two of the sides in a right triangle, you can find the dimensions for the missing side using the Pythagorean Theorem.
In a right triangle you know
the hypotenuse is 5 units long and one leg is 4 units long.
Calculate
the other leg: