Prove that ab squared + bc squared = ac squared

I remember doing this for homework, once. 
Imagine that we have a triangle
AB=3
BC=4
AC=5
So, we plug in
3^2 +4^2=5^2
9+16=25
25=25
WOAH! Say hello to the Pythagorean theorem.

What is so good about this formula. We only need to know 2 sides to find the other.
What if AB=BC
Let us try 1
1^2+1^2=c^2
1+1=c^2
2=c^2

Try finding a triangle with all sides revealed. Choose two sides(even the hypotenuse works) and plug it in. You will find that it works. If it does not, then either you did it wrong, or it is not a right triangle