10.设二维随机变量(X,Y)的联合概率密度为-|||-f(x,y)= ) c(R-sqrt ({x)^2+(y)^2}) . , ^2+(y)^2leqslant (R)^2,-|||-其他,-|||-求:(1)常数c; (2) {X)^2+(Y)^2leqslant (r)^2} (0lt rlt R).