9.设二维随机变量(X,Y)的联合密度函数如下:-|||-(1) (x,y)=dfrac (A)((1+{x)^2)(1+(y)^2)}(-infty lt x,ylt +infty ) :-|||-(2) f(x,y)= ) Axy,0lt xlt 1,0lt ylt 1 0, .-|||-,-|||-求:(1)未知常数A:(2)关于X,Y的边际密度函数.