The following is a proof that one equals two. Obviously there must be a flaw in it. When you think you've found it, select the line that results from the incorrect operation or assumption to see if you're right. If you don't have a browser that supports <SUP>, look at the proof on the right. Otherwise you can look at the one on the left.
For two numbers a and b, we want to prove:
a = b a = b
ab = b2 (line 2) ab = b^2
-(ab) = -(b2) (line 3) -(ab) = -(b^2)
a2-ab = a2-b2 (line 4) a^2-ab = a^2-b^2
a(a-b) = (a+b)(a-b) (line 5) a(a-b) = (a+b)(a-b)
a = a+b (line 6) a = a+b
a = 2a (line 7) a = 2a
1 = 2 1 = 2
Please note that I did not come up with this proof. It was passed on to me from someone who got it from one of his math professors.