Funny math

-20 = -20
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16-36 = 25-45
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16 - 36 + 81/4 = 25 - 45 + 81/4
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4^2 - 2*4*9/2 + (9/2)^2 = 5^2 - 2*5*9/2 + (9/2)^2
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(4 - 9/2)^2 = (5 - 9/2)^2
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4 - 9/2 = 5 - 9/2
4 = 5

2*2 = 5

where's mistake ?!..