4.设总体密度函数如下,x1,···,xn是样本,试求未知参数的矩估计.-|||-(1) (x;theta )=dfrac (2)({theta )^2}(theta -x) lt xlt theta ,θ>0;-|||-(2) (x;theta )=(theta +1)(x)^theta , lt xlt 1 ,θ>0;-|||-(3) (x;theta )=sqrt (theta )(x)^sqrt (theta -1) lt xlt 1 ,θ>0;-|||-(4) (x;theta ,mu )=dfrac (1)(theta )(e)^-dfrac (x-mu {theta )} gt mu ,θ>0.