Two random variables X and Y are distributed according to
fx,y(x,y)=((x,y),0≤x≤1,0≤y≤10,otherwise.
The probability P(X+Y≤1) is
P(x+y≤1)=∫Rf(x,y)dx dy
=∫10∫1−x0(x+y)dy dx
=∫10[xy+y22]1−x0dx
=∫10[x(1−x)+(1−x)22]dx
=[x22−x33−(1−x)36]10
=12−13+16
=13