题目:计算以xOy面上的圆周x^2+y^2=ax围成的闭区域为底,而已曲面z=x^2+y^2为顶的曲顶柱体的体积
解答:柱体的体积=2∫(0,π/2)dθ∫(0,acosθ)r^3dr
=1/2∫(0,π/2)(acosθ)^4dθ
=a^4/2∫(0,π/2)(acosθ)^4dθ
=a^4/8∫(0,π/2)[1+2cos(2θ)+cos²(2θ)]dθ
=a^4/8∫(0,π/2)[3/2+2cos(2θ)+cos(4θ)/2]dθ
=a^4/8[3θ/2+sin(2θ)+sin(4θ)/8]|(0,π/2)
=a^4/8(3π/4+0+0)
=3a^4/32.
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