y=x,x+y=1,x=0所形成的交点为((1/2,1/2),(1,0)
∫∫dxdy
=∫[0,1/2]dy∫[y,1-y]dx
=∫[0,1/2](1-2y)dy
=(y-y^2)[0,1/2]
=1/4