13.设总体X的概率分布律为 (x,theta )=(x-1)(theta )^2((1-theta ))^x-2 x=2 ,3,···; lt theta lt 1 为未知参数。若-|||-3,2,4,5,2,6是来自该总体的简单随机样本的观测值,则θ的矩估计值为 __