Hi, I've gotten Spivak's calculus and I have a question on the second proof in the first chapter What is wrong with the following "proof"? suppose x=y 1. x² = xy 2. x² - y²= xy - y² 3. (x + y)(x - y)=y(x - y) 4. x + y=y 5. 2y = y 6. 2 = 1 I just want to clarify that the error is in the transition from step three to step four as subtracting both sides by (x - y) is to subtract by zero as if x=y then x -y = 0. Step two is also saying that 0 = 0.