[17] Given that the nonnegative function $g(x)$ has the property that $$ \int_{0}^{\infty} g(x) d x=1. $$ Show that $$ f_{X, Y}(x, y)=\frac{2 g\left(\sqrt{x^{2}+y^{2}} ight)}{\pi \sqrt{x^{2}+y^{2}}}, \quad 0