Given: a = b
Multiply by a: a2 = ab
Subtract b2: a2 – b2 = ab – b2
Factorise: (a + b)(a – b) = b(a – b)
Divide by (a – b): (a + b) = b
Substitute for b: a + a = a
Simplify: 2a = a
Divide by a: 2 = 1

Clearly, this is impossible. So where’s the error?

Answers on a postcard, or in the comments