求二重积分
∫∫Dsin(x+y)dσ,其中D={(x,y)|0≤x≤π/2,0≤y≤π/2};
解:∫∫Dsin(x+y)dσ=∫∫Dsin(x+y)dxdy =∫π/20dx∫π/20sin(x+y)dy =∫π/20[-cos(x+y)]|π/20dx =∫π/20[cos-cos(x+π/2)]dx =∫π/20(cosx+sinx)dx =(sinx-cosx)|π/20=2
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