51. Let X and Y denote the coordinates of a point uniformly chosen in the disk of radius 1 centered at the origin. That is, their joint density is

$$

f(x,y) = \frac{1}{\pi}

$$

x² + y² ≤ 1 Find the joint density function of the polar coordinates R = (X² + Y²)¹/² and Θ = tan⁻¹Y/X.