Math/Logic Puzzles

Since we have a riddles thread, let's get a math/logic puzzle thread going!

I'll start. Here's a proof that 1=2. Let's see if you can spot the fallacy. (no google searches!)
let a = b
a^2 = ab
a^2 + a^2 = a^2 + ab
2a^2 = a^2 + ab
2a^2 - 2ab = a^2 + ab - 2ab
2a^2 - 2ab = a^2 - ab
2(a^2 - ab) = 1(a^2 - ab)
2 = 1

you should think about this one before looking at my answer.

Hint:

Answer:

Nice Vexer, seems like that was easy for you. Got any for me to try?

Lol,