利用极坐标计算∫∫xydxdy,其中D是第一象限中x+y=1与x+y=2x所围成的闭区域.答案是9/16

Question

利用极坐标计算∫∫xydxdy,其中D是第一象限中x+y=1与x+y=2x所围成的闭区域.
答案是9/16

Answer 1

x+y=1的极坐标方程为：r=1 x+y=2x的极坐标方程为：r=2rcosθ,即r=2cosθ 2cosθ=1,则：cosθ=1/2,θ=π/3 请自己画图 因此两曲线所围区域可分为两部分,第一部分θ：0-->π/3,r：0-->1 第二部分：θ：π/3-->π/2,r：0-->2cosθ ∫∫xydxdy =∫∫rcosθ*rsinθ*rdrdθ =∫[0-->π/3]∫[0-->1] rcosθsinθ drdθ+∫[π/3-->π/2]∫[0-->2cosθ] rcosθsinθ drdθ =∫[0-->π/3]cosθsinθdθ*∫[0-->1] rdr+∫[π/3-->π/2] cosθsinθdθ∫[0-->2cosθ] rdr =1/4∫[0-->π/3]sinθd(sinθ)*r | [0-->1] + 1/4∫[π/3-->π/2] cosθsinθ*r｜[0-->2cosθ]dθ =1/8sinθ |[0-->π/3] + 4∫[π/3-->π/2] cosθsinθ*cosθdθ =(1/8)(3/4) - 4∫[π/3-->π/2] cosθd(cosθ) =3/32 - 2/3cosθ |[π/3-->π/2] =3/32 + (2/3)(1/64) =9/96 + 1/96 =10/96 =5/48 感觉这个题应该是你的答案错了.