(1).Sn=1+2×3+3×7...n(2^n-1),求Sn.(2).已知数列{an}中,an=-2[n-(-1)^n],求Sn.(3).求数列 1,a+a^2,a^2+a^3+a^4,a^3+a^4+a^5+a^6...,的前n项和.
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