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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=lnxy = \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

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  1. xx 軸旋轉之體積使用墊圈法(Washer method): V=12π[(yupper)2(ylower)2]dx=π12[x(lnx)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 (lnx)2dx\int (\ln x)^2\,\mathrm{d}x

答題過程

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使用墊圈法:

V=π12[(x)2(lnx)2]dx=π12[x(lnx)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:

12xdx=[x22]12=212=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):u=(lnx)2,dv=dx    du=2lnxxdx,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

(lnx)2dx=x(lnx)22lnxdx=x(lnx)22(xlnxx)=x(lnx)22xlnx+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(lnx)2dx=[x(lnx)22xlnx+2x]12=(2(ln2)24ln2+4)(00+2)=2(ln21)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[322(ln21)2]V = \pi \left[ \frac{3}{2} - 2(\ln 2 - 1)^2 \right]

故 (9) 處應填入 π[322(ln21)2]\displaystyle\boxed{\pi \left[ \frac{3}{2} - 2(\ln 2 - 1)^2 \right]}


(b)

解法一

思路

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  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)=4xr(x) = 4-x,高度為 h(x)=xlnxh(x) = \sqrt{x} - \ln x
  2. 展開被積函數並分項積分。
  3. (4x)lnxdx\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(4x)(xlnx)dx=2π12[4x1/2x3/2(4x)lnx]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

    124x1/2dx=4[23x3/2]12=83(221)=162383\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

    12x3/2dx=[25x5/2]12=25(421)=82525\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 (4x)lnxdx\int (4-x)\ln x\,\mathrm{d}x。令 u=lnx,dv=(4x)dx    du=1xdx,v=4xx22u = \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}

    (4x)lnxdx=(4xx22)lnx(4x2)dx=(4xx22)lnx(4xx24)\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(4x)lnxdx=[(82)ln2(81)][0(41/4)]=(6ln27)+154=6ln2134\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[(162383)(82525)(6ln2134)]=2π[2(16385)83+25+1346ln2]=2π[56215+59606ln2]=π30[59+2242360ln2]\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+2242360ln2]\displaystyle\boxed{\frac{\pi}{30} \left[ 59 + 224\sqrt{2} - 360\ln 2 \right]}