Theorem: 4 = 5
Proof:
-20 = -20
16 - 36 = 25 - 45
4^2 - 9*4 = 5^2 - 9*5
4^2 - 9*4 + 81/4 = 5^2 - 9*5 + 81/4
(4 - 9/2)^2 = (5 - 9/2)^2
4 - 9/2 = 5 - 9/2
4 = 5

Genius!

Actually this is wrong, here's why:

4 - 9/2 != 5 - 9/2
4 - 4.5 != 5 - 4.5
-0.5 != 0.5

The only reason (4 - 9/2)^2 = (5 - 9/2)^2 is because the equation is squared when you work it out:

(4 - 9/2)^2 = (5 - 9/2)^2
(4 - 4.5)^2 = (5 - 4.5)^2
(-0.5)^2 = (0.5)^2
0.25 = 0.25

hmm... ok. dang, Andy B, my patna dem! you just had to show me up, huh? lol...

Yes, because 4 can not equal 5