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理论上能算出来,但很麻烦。这类题用柱坐标简单。
x^2+y^2 = r^2 ≤ z
I = ∫<0, 2π>dt∫<0, √6>rdr∫<r^2, 6>zdz
= 2π∫<0, √6>rdr[z^2/2]<r^2, 6>
= π∫<0, √6>(36r-r^5)dr = π[18r^2-r^6/6]<0, √6> = 72π
或 I = ∫<0, 6>zdz∫<0, 2π>dt∫<0, √z>rdr
= 2π∫<0, 6>zdz[r^2/2]<0, √z>
= π∫<0, 6>z^2dz = (π/3)[z^3]<0, 6> = 72π
x^2+y^2 = r^2 ≤ z
I = ∫<0, 2π>dt∫<0, √6>rdr∫<r^2, 6>zdz
= 2π∫<0, √6>rdr[z^2/2]<r^2, 6>
= π∫<0, √6>(36r-r^5)dr = π[18r^2-r^6/6]<0, √6> = 72π
或 I = ∫<0, 6>zdz∫<0, 2π>dt∫<0, √z>rdr
= 2π∫<0, 6>zdz[r^2/2]<0, √z>
= π∫<0, 6>z^2dz = (π/3)[z^3]<0, 6> = 72π
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