2^0+2^1+2^2+2^3+2^4+……+2^n=2^0+2^0+2^1+2^2+2^3+2^4+……+2^n-2^0=2^1+2^1+2^2+2^3+2^4+……+2^n-2^0=2^2+2^2+2^3+2^4+……+2^n-2^0=....=2^(n+1)-1
等比数列（1-2^(n+1)）/(1-2)=2^(n+1)-1
