①已知数列{an}的an=(-1)n次方(2n-1)求数列an前40项和.②求和:sn=1/2×4+1/3×5+1/4×6+.+1/(n+1)(n+3)③数列1,(1+2),(1+2+2²),.,(1+2+2²+...+2的n-1次方)...(1)求an通项公式(2)求前n项和sn.
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![①已知数列{an}的an=(-1)n次方(2n-1)求数列an前40项和.②求和:sn=1/2×4+1/3×5+1/4×6+.+1/(n+1)(n+3)③数列1,(1+2),(1+2+2²),.,(1+2+2²+...+2的n-1次方)...(1)求an通项公式(2)求前n项和sn.](/uploads/image/z/1158316-52-6.jpg?t=%E2%91%A0%E5%B7%B2%E7%9F%A5%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84an%3D%28-1%29n%E6%AC%A1%E6%96%B9%282n-1%29%E6%B1%82%E6%95%B0%E5%88%97an%E5%89%8D40%E9%A1%B9%E5%92%8C.%E2%91%A1%E6%B1%82%E5%92%8C%3Asn%3D1%2F2%C3%974%2B1%2F3%C3%975%2B1%2F4%C3%976%2B.%2B1%2F%28n%2B1%29%28n%2B3%29%E2%91%A2%E6%95%B0%E5%88%971%2C%EF%BC%881%2B2%EF%BC%89%2C%EF%BC%881%2B2%2B2%26%23178%3B%EF%BC%89%2C.%2C%EF%BC%881%2B2%2B2%26%23178%3B%2B...%2B2%E7%9A%84n-1%E6%AC%A1%E6%96%B9%EF%BC%89...%EF%BC%881%EF%BC%89%E6%B1%82an%E9%80%9A%E9%A1%B9%E5%85%AC%E5%BC%8F%EF%BC%882%EF%BC%89%E6%B1%82%E5%89%8Dn%E9%A1%B9%E5%92%8Csn.)
①已知数列{an}的an=(-1)n次方(2n-1)求数列an前40项和.②求和:sn=1/2×4+1/3×5+1/4×6+.+1/(n+1)(n+3)③数列1,(1+2),(1+2+2²),.,(1+2+2²+...+2的n-1次方)...(1)求an通项公式(2)求前n项和sn.
①已知数列{an}的an=(-1)n次方(2n-1)求数列an前40项和.②求和:sn=1/2×4+1/3×5+1/4×6
+.+1/(n+1)(n+3)③数列1,(1+2),(1+2+2²),.,(1+2+2²+...+2的n-1次方)...(1)求an通项公式(2)求前n项和sn.
①已知数列{an}的an=(-1)n次方(2n-1)求数列an前40项和.②求和:sn=1/2×4+1/3×5+1/4×6+.+1/(n+1)(n+3)③数列1,(1+2),(1+2+2²),.,(1+2+2²+...+2的n-1次方)...(1)求an通项公式(2)求前n项和sn.
①an=(-1)^n*(2n-1),
数列an前40项和=(-1+3)+(-5+7)+……+(-77+79)
=2*20=40.
②1/[(n+1)(n+3)]=(1/2)[1/(n+1)-1+3)],
∴Sn=(1/2)[1/2-1/4+1/3-1/5+1/4-1/6+……+1/(n+1)-1/(n+3)]
=(1/2)[1/2+1/3-1/(n+2)-1/(n+3)]
=5/12-(2n+5)/[2(n+2)(n+3)].
③(1)an=1+2+2^2+……+2^(n-1)=2^n-1.
(2)Sn=2^(n+1)-2-n.
已知数列{an}的an=(-1)n次方(2n-1)
表达清楚下好吗,已知的到底是什么