| 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