sn=2n^2.bn为等比数列.且a1=b1.b2*(a2-a1)=b1.求an和bn?
问题描述:
sn=2n^2.bn为等比数列.且a1=b1.b2*(a2-a1)=b1.求an和bn?
答
Sn=2n^2S(n-1)=2(n-1)^2an=Sn-S(n-1)=2n^2-2(n-1)^2=2(2n-1)=4n-2b1=a1=4-2=2b2=b1/(a2-a1)=2/4=1/2q=b2/b1=1/4bn=b1q^(n-1)=2*(1/4)^(n-1)=2*(1/2)^(2n-2)=(1/2)^(2n-3)