1. (10 points) Let D = {(x,y) | x² + y² ≤ 1} be the unit disc. Consider the coordinate in the polar
system. Let r~ Uniform([0, 1]) and θ ~ Uniform([0, 2π)), sampled from two independent uniform
distributions. For
x = r cos θ,
y = r sin θ,
find the joint distribution function fx,y (x, y) of (X, Y).