f(x,y)=A(R-√(x^2+y^2)),x^2+y^2<=R^2;f(x,y)=0，其他由∫f(x,y)dx=1得(AπR^3)/3=1,所以A=3/(πR^3)(X,Y)落入圆x^2+y^2<=r^2(r即求f(x,y)在x^2+y^2<=r^2(r积分得：(X,Y)落入圆x^2+y^2<=r^2(r解毕
