先积x
∫∫(x+y)dxdy
=∫[0→1]dy∫[0→y](x+y)dx
=∫[0→1] [(1/2)x²+xy] |[0→y] dy
=∫[0→1] (3/2)y² dy
=(1/2)y³ |[0→1]
=1/2