Spot the Mathematical Fallacy

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Which line contains the fallacy in this mathematical proof?

1. Let a and b be equal non-zero quantities.
a = b

2. Multiply both sides by a.
a2 = ab

3. Subtract b2.
a2 - b2 = ab - b2

4. Factor both sides.
(a - b)(a + b) = b(a - b)

5. Divide out (a - b).
(a + b) = b

6. Substitute a = b.
b + b = b

7. Simplify.
2b = b

8. Divide by b.
2 = 1

Very famous and illuminating example.