设数列{An}为等差数列,且a5=14,a7=20.数列{Bn}的前n项和为Sn=(1/3)n (n

问题描述:

设数列{An}为等差数列,且a5=14,a7=20.数列{Bn}的前n项和为Sn=(1/3)n (n

公差为(a7-a5)/2=3,