换到极坐标系，积分区域D：0≤ θ ≤ π/2, 0≤ p ≤ 2a cosθ
I = ∫[0,π/2] dθ ∫ [0, 2a cosθ] p^3 dp
= ∫[0,π/2] 4 a^4 (cosθ)^4 dθ
= 4 a^4 * (3π/16)
= 3π a^4 /4
换成极坐标系，I＝∫(0,π/2)dθ∫(0,2acosθ) ρ^3dρ＝∫(0,π/2) 4a^4(cosθ)^4dθ＝4a^4×3/4×1/2×π/2＝πa^4/4
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