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设an是fn(x)＝(1＋x)n+1(x∈N*)的展开式中xn项的系数，则an＝________；数列{an}的前n项和为________．
设an是fn(x)＝(1＋x)n+1(x∈N*)的展开式中xn项的系数，则an＝________；数列{an}的前项和为________．
项和为________．