Question

Let a = b

a^2 = ab
a^2-b^2 = ab - b^2
(a+b)(a-b) = b(a-b)
a+b = b
a+a = a
2a = a
2 = 1

Explain.

Answer 1

You're dividing by zero on step 3...
\[(a+b)(a-b)=b(a-b)\\a+b=b\]
The above algebraic transformation is only well-defined for where...\[a-b\neq0\]