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3n-m
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1/2 + 1/4 + 1/8 + 1/16 + ... = 1.
4 + 1/4 + 1/8 + 1/16 + 1/32 + ... = 4 - 1/2 + 1/2 + 1/4 + 1/8 + 1/16 + ... = 4 - 1/2 + 1 = 4 + 1/2