Algebra

a=b

Multiply by a:

a²=ab

Subtract b²:

a²-b²=ab-b²

Factor:

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

Divide by (a-b):

a+b=b

As a=b, substitute:

b+b=b

Combine:

2b=b

Divide by b:

2=1

Question. What's wrong?