两个等差数列{an},{bn}的前n项之和分别为An和Bn,且An/Bn=(7n+2)/(7n+3),求a7/b7的值.加油加油~>.

Question

两个等差数列{an},{bn}的前n项之和分别为An和Bn,且An/Bn = (7n+2)/(7n+3),求a7/b7的值.
加油 加油~>.

Answer 1

An/Bn =[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)]
=(7n+2)/(7n+3)
所以d=d'=7,a1=9/2,b1=5
a7/b7=(a1+6d)/(b1+6d')=46/47