# $$9=11$$!!!

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Line 1: $$99=99$$

Line 2: $$180-81=220-121$$

Line 3: $$81-180=121-220$$

Line 4: $$9^2-2\cdot9\cdot10+10^2=11^2-2\cdot11\cdot10+10^2$$

Line 5: $$(9-10)^2=(11-10)^2$$

Line 6: $$9-10=11-10$$

Cancelling $$-10$$ from both sides, we obtain $$9=11$$

In which line I did the mistake?

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