∫∫Dcos(x+y)dσ，其中D是由x=0，y=π，y=x所围区域．
∫∫Dcos(x+y)dσ=∫0πdx∫xπ(x+y)dy= ∫0πsin(x+y)∣xπdx =∫0π[sin(x+π)-sin2x]dx =-cos(x+π)∣0π+(1/2)cos2x∣0π=-2
∫∫Dcos(x+y)dσ，其中D是由x=0，y=π，y=x所围区域．
∫∫Dcos(x+y)dσ=∫0πdx∫xπ(x+y)dy= ∫0πsin(x+y)∣xπdx =∫0π[sin(x+π)-sin2x]dx =-cos(x+π)∣0π+(1/2)cos2x∣0π=-2
