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