证明： 2^0+2^1+2^2+2^3+.+2^n-1=(2^n)-1
S=2^0+2^1+2^2+2^3+.+2^(n-1)
2S=2^1+2^2+2^3+...+2^(n-1)+2^n
两式相减,
2S-S=2^n-2^0
S=2^(n)-1
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S=2^0+2^1+2^2+2^3+.+2^(n-1)
2S=2^1+2^2+2^3+...+2^(n-1)+2^n
两式相减,
2S-S=2^n-2^0
S=2^(n)-1
