p(x,y)=p(x)p(y∣x)p(x, y)=p(x) p(y \mid x)p(x,y)=p(x)p(y∣x) by Chain Rule (Probability).
By the Chain Rule (Probability), the joint distribution of (x,y)(x, y)(x,y) is p(x,y)p(x, y)p(x,y).
Chain Rule (Probability)