Thread: 2=1 ?

Hello,

I have a question regarding a problem that my prof gave me:

x=y
then
x² = xy
x²- y²= xy- y²
(x-y) (x+y) = (x-y) y
x+y= y
y+y=y
2y=y
which would mean that 2=1

Can someone explain me where the problem is?
Thanks in adavance!

Originally Posted by chrisjohnston

(x-y) (x+y) = (x-y) y
x+y= y

Hi

This is a well known problem
You cannot divide by (x-y) since it is equal to 0

Mathematical fallacy - Wikipedia, the free encyclopedia

Scroll to Division by zero for your question.

Thanks for your quick replies guys!!!!