有两个等差数列{an}{bn},若(a1+a2+.+an)/(b1+b2+.+bn)=(3n-1)/(2n+3)则a13/b13=?
问题描述：
有两个等差数列{an}{bn},若(a1+a2+.+an)/(b1+b2+.+bn)=(3n-1)/(2n+3)则a13/b13=?
答
设数列{an}前n项和为Sn,公差为d；数列{bn}前n项和为Tn,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']=[(2a1-d)+nd]/[(2b1-d')+nd']=(a1+a2+...+an)/(b1+b2+...+bn)=(3n-1)/(2n+3)令d...