Math Help -> Jokes -> Proof that 1 = 2

Proof that 2 = 1

let a = b

a² = ab

Multiply both sides by a

a² + a² - 2ab = ab + a² - 2ab

Add (a² - 2ab) to both sides

2(a² - ab) = a² - ab

Factor the left, and collect like terms on the right

2 = 1

Divide both sides by (a² - ab)

This proof can also be used as a proof by contradiction that a can never be equal to b.

Index of joke proofs

An alternative proof that 2=1

A proof by induction that e = π

The webmaster and author of the Math Help site is Graeme McRae.
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