Question

等比数列an的前n项和Sn=2的n次方 -1,则a1的平方+a2的平方+a3的平方+……an的平方=?

Answer 1

a1 = s1 = 2^1 - 1 = 1;

a2 = s2 - s1 = 2^2 -1 - 1 = 2

等比b = a2/a1 = 2

这样,a1^2 ,a2^2.an^2形成了等比为4的等比数列.则答案为：

-(1-4^n)/3