- 679 名前：１３２人目の素数さん [2022/09/23(金) 19:44:36.14 ID:N15NgvLO.net]
- >>495
(i)回転体をx=t(-1/2≦t＜0,1＜t≦3/2)で切った断面はドーナツ型で、
体積=2π〔∫[t=-1/2→0][√3/2+√｛1-(1/2-t)^2｝]^2-∫[t=1→3/2][√3/2-√｛1-(1/2-t)^2｝]^2〕
=4π√3∫[t=-1/2→0]√｛1-(1/2-t)^2｝dt
1/2-t=cosθとおくと-dt=-sinθdθ
dt=sinθdθ
体積=2π√3∫[θ=0→π/3]sinθsinθdθ
=4π√3∫[θ=0→π/3]sinθ^2θdθ
=4π√3∫[θ=0→π/3](1/2-cos2θ/2)dθ
=4π√3[θ=0→π/3][θ/2-sin2θ/4]dθ
=4π√3(π/6-√3/8)
=2π^2√3/3-3π/2
体積=π∫[θ=π/3→2π/3](√3/2+sinθ)^2dθ
=π∫[θ=π/3→2π/3](3/4+sinθ√3+sin^2θ)dθ
=π[θ=π/3→2π/3][3θ/4-cosθ√3+θ/2-sin2θ/4]
=π[θ=π/3→2π/3][5θ/4-cosθ√3-sin2θ/4]
=π[5π/6-(-1/2)√3-(-√3/8)-｛5π/12-(1/2)√3-(√3/8)｝]
=π(5π/12+√3+√3/4)
=5π^2/12+5π√3/4
(i)(ii)より、
体積=2π^2√3/3-3π/2+5π^2/12+5π√3/4
=(5+8√3)π^2/12+(5√3-6)π/4
=17.5981313181……
π^2√3より大
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