Can you spot the mistake?

Given a = b

then we can derive as follows:

$a^{2}$ = $a \times b$

$a^{2} - b^{2}$ = $ab - b^{2}$

(a + b) (a - b) = b (a - b)

(a + b) = b

(b + b) = b

2b = b

2 = 1 !