已知两个等差数列{An} {bn},他们的前n项和分别是Sn,Tn ,若Sn/Tn=3n-1/2n+3,求a9/b7
问题描述：
已知两个等差数列{An} {bn},他们的前n项和分别是Sn,Tn ,若Sn/Tn=3n-1/2n+3,求a9/b7
答
设{an}公差为d,{bn}公差为d'Sn/Tn=[na1+n(n-1)d/2]/[nb1+n(n-1)d'/2]=[2a1+(n-1)d]/[2b1+(n-1)d']=[dn+(2a1-d)]/[d'n+(2b1-d')]=(3n-1)/(2n+3)令d=3t,则2a1-d=-t,d'=2t,2b1-d'=3t解得a1=t d=3t b1=(5/2)t d'=2ta9/b7...