1.数列:1,-2,3,-4,…,(-1)^(n-1)的前99项和为多少?2.等比数列1,a1,a2,a3,…,a2n,2共有2n+2项,则a1*a2*a3*…*a2n=?
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![1.数列:1,-2,3,-4,…,(-1)^(n-1)的前99项和为多少?2.等比数列1,a1,a2,a3,…,a2n,2共有2n+2项,则a1*a2*a3*…*a2n=?](/uploads/image/z/1072070-62-0.jpg?t=1.%E6%95%B0%E5%88%97%EF%BC%9A1%2C-2%2C3%2C-4%2C%E2%80%A6%2C%EF%BC%88-1%EF%BC%89%5E%28n-1%29%E7%9A%84%E5%89%8D99%E9%A1%B9%E5%92%8C%E4%B8%BA%E5%A4%9A%E5%B0%91%3F2.%E7%AD%89%E6%AF%94%E6%95%B0%E5%88%971%2Ca1%2Ca2%2Ca3%2C%E2%80%A6%2Ca2n%2C2%E5%85%B1%E6%9C%892n%2B2%E9%A1%B9%2C%E5%88%99a1%2Aa2%2Aa3%2A%E2%80%A6%2Aa2n%3D%3F)
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1.数列:1,-2,3,-4,…,(-1)^(n-1)的前99项和为多少?2.等比数列1,a1,a2,a3,…,a2n,2共有2n+2项,则a1*a2*a3*…*a2n=?
1.数列:1,-2,3,-4,…,(-1)^(n-1)的前99项和为多少?
2.等比数列1,a1,a2,a3,…,a2n,2共有2n+2项,则a1*a2*a3*…*a2n=?
1.数列:1,-2,3,-4,…,(-1)^(n-1)的前99项和为多少?2.等比数列1,a1,a2,a3,…,a2n,2共有2n+2项,则a1*a2*a3*…*a2n=?
1.数列:1,-2,…,(-1)^(n-1)的前99项和
=(1-2)+(3-4)+...+(97-98)+99
=-98/2+99
=50
2.等比数列1,a1,a2,a3,…,a2n,2共有2n+2项
a1*a2*a3*…*a2n=1×q^(1+2+...+2n)=q^2n(2n+1)/2=q^n(2n+1)
2=1×q^(2n+1)=q^(2n+1)
所以a1*a2*a3*…*a2n=q^n(2n+1)=[q^(2n+1)]^n=2^n
1.(1+3+...+99)-(2+4+...+98)=50
1
两项两项合并1-2=-1,3-4=-1,,所以前98项和为-49
第99项等于99,,所以和为-49+99=50