等差数列与等比数列混合,用错位相减法.设原式=s,则2s=2+3/2^1+4/2^2+...+n/2^(n-2)+(n+1)/2^(n-1),s=2s-s=2-(n+1)/2^n+(1/2^1+1/2^2+1/2^3+...+1/2^(n-1))=2-(n+1)/2^n+(1-1/2^(n-1))=3-(n+3)/2^n....