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111 台灣大學微積分(C) 第 5 題

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111學年度 · 111台大微積分C · 第 5 題

題目

Problem

5. Let RR be the region under y=xy = \sqrt{x}, above y=ln⁡xy = \ln x, and between x=1x=1 and x=2x=2. (10%)

Find the volume of the solid obtained by rotating RR about the xx-axis. (9)‾\underline{\quad (9) \quad}

Find the volume of the solid obtained by rotating RR about the line x=4x = 4. (10)‾\underline{\quad (10) \quad}

解答

(a)

解法一

思路

展開
  1. 繞 xx 軸旋轉之體積使用墊圈法(Washer method): V=∫12π[(yupper)2−(ylower)2]dx=π∫12[x−(ln⁡x)2]dxV = \int_1^2 \pi \left[ (y_{\text{upper}})^2 - (y_{\text{lower}})^2 \right] \mathrm{d}x = \pi \int_1^2 \left[ x - (\ln x)^2 \right] \mathrm{d}x
  2. 分開積分,並用分部積分法求得 ∫(ln⁡x)2 dx\int (\ln x)^2\,\mathrm{d}x。

答題過程

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使用墊圈法:

V=π∫12[(x)2−(ln⁡x)2]dx=π∫12[x−(ln⁡x)2]dxV = \pi \int_1^2 \left[ (\sqrt{x})^2 - (\ln x)^2 \right] \mathrm{d}x = \pi \int_1^2 \left[ x - (\ln x)^2 \right] \mathrm{d}x

計算第一部分:

∫12x dx=[x22]12=2−12=32\int_1^2 x\,\mathrm{d}x = \left[ \frac{x^2}{2} \right]_1^2 = 2 - \frac{1}{2} = \frac{3}{2}

計算第二部分(使用分部積分): 令 u=(ln⁡x)2,dv=dx  ⟹  du=2ln⁡xx dx,v=xu = (\ln x)^2, \mathrm{d}v = \mathrm{d}x \implies \mathrm{d}u = \frac{2\ln x}{x}\,\mathrm{d}x, v = x:

∫(ln⁡x)2 dx=x(ln⁡x)2−2∫ln⁡x dx=x(ln⁡x)2−2(xln⁡x−x)=x(ln⁡x)2−2xln⁡x+2x\int (\ln x)^2\,\mathrm{d}x = x(\ln x)^2 - 2\int \ln x\,\mathrm{d}x = x(\ln x)^2 - 2(x\ln x - x) = x(\ln x)^2 - 2x\ln x + 2x

帶入積分限 [1,2][1, 2]:

∫12(ln⁡x)2 dx=[x(ln⁡x)2−2xln⁡x+2x]12=(2(ln⁡2)2−4ln⁡2+4)−(0−0+2)=2(ln⁡2−1)2\int_1^2 (\ln x)^2\,\mathrm{d}x = \left[ x(\ln x)^2 - 2x\ln x + 2x \right]_1^2 = \left( 2(\ln 2)^2 - 4\ln 2 + 4 \right) - (0 - 0 + 2) = 2(\ln 2 - 1)^2

合併結果:

V=π[32−2(ln⁡2−1)2]V = \pi \left[ \frac{3}{2} - 2(\ln 2 - 1)^2 \right]

故 (9) 處應填入 π[32−2(ln⁡2−1)2]\displaystyle\boxed{\pi \left[ \frac{3}{2} - 2(\ln 2 - 1)^2 \right]}。


(b)

解法一

思路

展開
  1. 繞平行 yy 軸的直線 x=4x=4 旋轉,採用圓柱殼法(Cylindrical shells method)最為便利: V=∫122πr(x)h(x) dxV = \int_1^2 2\pi r(x) h(x)\,\mathrm{d}x 這裡半徑為 r(x)=4−xr(x) = 4-x,高度為 h(x)=x−ln⁡xh(x) = \sqrt{x} - \ln x。
  2. 展開被積函數並分項積分。
  3. ∫(4−x)ln⁡x dx\int (4-x)\ln x\,\mathrm{d}x 使用分部積分法求解。

答題過程

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使用圓柱殼法:

V=∫122π(4−x)(x−ln⁡x) dx=2π∫12[4x1/2−x3/2−(4−x)ln⁡x]dxV = \int_1^2 2\pi (4-x)(\sqrt{x} - \ln x)\,\mathrm{d}x = 2\pi \int_1^2 \left[ 4x^{1/2} - x^{3/2} - (4-x)\ln x \right] \mathrm{d}x

我們將其分為三部分積分:

  1. 第一項:

    ∫124x1/2 dx=4[23x3/2]12=83(22−1)=1623−83\int_1^2 4x^{1/2}\,\mathrm{d}x = 4 \left[ \frac{2}{3}x^{3/2} \right]_1^2 = \frac{8}{3}(2\sqrt{2}-1) = \frac{16\sqrt{2}}{3} - \frac{8}{3}
  2. 第二項:

    ∫12x3/2 dx=[25x5/2]12=25(42−1)=825−25\int_1^2 x^{3/2}\,\mathrm{d}x = \left[ \frac{2}{5}x^{5/2} \right]_1^2 = \frac{2}{5}(4\sqrt{2}-1) = \frac{8\sqrt{2}}{5} - \frac{2}{5}
  3. 第三項(使用分部積分): 求不定積分 ∫(4−x)ln⁡x dx\int (4-x)\ln x\,\mathrm{d}x。令 u=ln⁡x,dv=(4−x) dx  ⟹  du=1x dx,v=4x−x22u = \ln x, \mathrm{d}v = (4-x)\,\mathrm{d}x \implies \mathrm{d}u = \frac{1}{x}\,\mathrm{d}x, v = 4x - \frac{x^2}{2}:

    ∫(4−x)ln⁡x dx=(4x−x22)ln⁡x−∫(4−x2)dx=(4x−x22)ln⁡x−(4x−x24)\int (4-x)\ln x\,\mathrm{d}x = \left( 4x - \frac{x^2}{2} \right)\ln x - \int \left(4 - \frac{x}{2}\right) \mathrm{d}x = \left( 4x - \frac{x^2}{2} \right)\ln x - \left( 4x - \frac{x^2}{4} \right)

    代入積分限 [1,2][1, 2]:

    ∫12(4−x)ln⁡x dx=[(8−2)ln⁡2−(8−1)]−[0−(4−1/4)]=(6ln⁡2−7)+154=6ln⁡2−134\int_1^2 (4-x)\ln x\,\mathrm{d}x = \left[ (8-2)\ln 2 - (8-1) \right] - \left[ 0 - (4-1/4) \right] = (6\ln 2 - 7) + \frac{15}{4} = 6\ln 2 - \frac{13}{4}

合併全部積分結果:

V= 2π[(1623−83)−(825−25)−(6ln⁡2−134)]= 2π[2(163−85)−83+25+134−6ln⁡2]= 2π[56215+5960−6ln⁡2]= π30[59+2242−360ln⁡2]\begin{align*} V =&\, 2\pi \left[ \left( \frac{16\sqrt{2}}{3} - \frac{8}{3} \right) - \left( \frac{8\sqrt{2}}{5} - \frac{2}{5} \right) - \left( 6\ln 2 - \frac{13}{4} \right) \right] \\[4mm] =&\, 2\pi \left[ \sqrt{2}\left(\frac{16}{3} - \frac{8}{5}\right) - \frac{8}{3} + \frac{2}{5} + \frac{13}{4} - 6\ln 2 \right] \\[4mm] =&\, 2\pi \left[ \frac{56\sqrt{2}}{15} + \frac{59}{60} - 6\ln 2 \right] \\[4mm] =&\, \frac{\pi}{30} \left[ 59 + 224\sqrt{2} - 360\ln 2 \right] \end{align*}

故 (10) 處應填入 π30[59+2242−360ln⁡2]\displaystyle\boxed{\frac{\pi}{30} \left[ 59 + 224\sqrt{2} - 360\ln 2 \right]}。