设曲线L:(0≤t≤π/2)与l:x2+y2≤1(x≥0,y≥0)所围区域为D。计算I=(x2-y2+1)dxdy

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问题 设曲线L:(0≤t≤π/2)与l:x2+y2≤1(x≥0,y≥0)所围区域为D。计算I=(x2-y2+1)dxdy

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答案D如图所示,由于D关于直线y=x对称,故 [*](x2-y2)=1/2[*](x2-y2+y2-x2)dxdy=0, 所以I=[*]dxdy 记D1为曲线L及坐标轴所围区域,则 I=[*]dxdy-[*]dxdy=1/4 π×12-[*]dxdy 其中 [*]dxdy=∫01dx[*]dy=∫01y[t(x)]dx [*]y(t)d[x(t)]=∫π/20sin3t·3cos3t(-sint)dt =3∫0π/2sin4tcos2tdt=∫0π/2sin4(1-sin2)dt=3π/32 故I=π/4-3π/32=5π/32 [*]

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