设等差数列an的前n项和为sn,已知a5=-3,S10=-40,若数列 a(bn)为等比数列,且b1=5,b2=8
问题描述:
设等差数列an的前n项和为sn,已知a5=-3,S10=-40,若数列 a(bn)为等比数列,且b1=5,b2=8
求数列bn的前n项和Tn
答
a5=-3,S10=-40Sn=na1+n(n-1)d/2a5=a1+4d=-3s10=10a1+45*d=-402a1+9d=-82a1+8d=-6 (a1+4d=-3)d=-2a1=5an=5+(n-1)*(-2)=7-2na(b1)=a5=-3a(b2)=a8=-9q=3a(bn)=a(b1)q^(n-1)=-3*3^(n-1)=-3^n=7-2bnbn=(7+3^n)/2Tn=7n/2+3...