∫∫x*cos(x+y) dy dx x=0→π,y=0→x
= ∫[x*sin(x+x) - xsinx] dx
= ∫[x*sin(2x) - xsinx] dx
= - xcos(2x)/2 + sin2x/4 + xcosx - sinx x=0→π
= - π/2 - π
= - 3π/2