Let XXX and YYY be independent with fX(x)=2xf_X(x) = 2xfX(x)=2x for 0≤x≤10 \le x \le 10≤x≤1 and fY(y)=2yf_Y(y) = 2yfY(y)=2y for 0≤y≤10 \le y \le 10≤y≤1. What is the joint PDF?
f(x,y)=4xyf(x,y) = 4xyf(x,y)=4xy
f(x,y)=2x+2yf(x,y) = 2x + 2yf(x,y)=2x+2y
f(x,y)=2f(x,y) = 2f(x,y)=2
f(x,y)=x+yf(x,y) = x + yf(x,y)=x+y