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 ,theta gt 0 ;-|||-(2) (x;theta )=(theta +1)(x)^theta ,lt xlt 1 ,theta gt 0 ;-|||-(3) (x;theta )=sqrt (theta )(x)^sqrt (theta -1) ,lt xlt 1 ,theta gt 0 ;-|||-(4) (x;theta ,mu )=dfrac (1)(theta )(e)^-dfrac (x-mu {theta )} ,gt mu ,theta gt 0 .