a = b <--- Set a equal to b a^2 = ab <--- Multiply each side by a a^2 - b^2 = ab - b^2 <--- Subtract b^2 from each side (a + b)(a - b) = b(a - b) <--- Factor each side a + b = b <--- Each side has (a - b) in common, so divide each side by (a - b) b + b = b <---Replace the a with b, since a = b 2b = b <--- Combine like terms 2 = 1 <--- Divide each side by b to get 2 = 1 See if you can spot the error made here, noting in which line the error occurs, and what the error is. It took me a while to figure it out when I first saw it in 9th grade, but someone will most likely figure it out pretty quickly. EDIT: Zeal sucks at math lolol. He thinks anything divided by zero equals infinity