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111 學年度台聯大微積分 A3/A4/A6 第 3 題

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111學年度 · 111台聯大微積分A3/A4/A6 · 第 3 題

題目

Problem

3. Evaluate 4x23x+24x24x+3dx\int \frac{4x^2 - 3x + 2}{4x^2 - 4x + 3}\,\mathrm{d}x.

解答

解法一:多項式除法與湊項法

思路

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  1. 此為有理函數積分。由於分子與分母的最高次項皆為 2 次,先進行多項式除法: 4x23x+24x24x+3=1+x14x24x+3\frac{4x^2 - 3x + 2}{4x^2 - 4x + 3} = 1 + \frac{x - 1}{4x^2 - 4x + 3}
  2. 將餘式項拆分。分母為二次不可約多項式,其導數為 8x48x-4。 將分子湊成包含分母導數的項: x1=18(8x4)12x - 1 = \frac{1}{8}(8x - 4) - \frac{1}{2}
  3. 拆開為兩項積分:
    • 第一項: 188x44x24x+3dx=18ln(4x24x+3)\frac{1}{8} \int \frac{8x-4}{4x^2-4x+3}\,\mathrm{d}x = \frac{1}{8}\ln(4x^2-4x+3)
    • 第二項:將分母配方,使用對應的 tan1\tan^{-1} 三角反函數公式求解。

答題過程

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第一步:多項式除法與化簡

4x23x+24x24x+3dx=(1+x14x24x+3)dx=x+x14x24x+3dx\int \frac{4x^2 - 3x + 2}{4x^2 - 4x + 3}\,\mathrm{d}x = \int \left( 1 + \frac{x - 1}{4x^2 - 4x + 3} \right) \mathrm{d}x = x + \int \frac{x - 1}{4x^2 - 4x + 3}\,\mathrm{d}x

第二步:分子拆分與湊導數

觀察分母的微分為:

ddx(4x24x+3)=8x4\frac{\mathrm{d}}{\mathrm{d}x}(4x^2 - 4x + 3) = 8x - 4

將分子 x1x-1 改寫為含有 8x48x-4 的形式:

x1=18(8x4)12x - 1 = \frac{1}{8}(8x - 4) - \frac{1}{2}

則待求積分項可拆為:

x14x24x+3dx=188x44x24x+3dx1214x24x+3dx\int \frac{x - 1}{4x^2 - 4x + 3}\,\mathrm{d}x = \frac{1}{8} \int \frac{8x - 4}{4x^2 - 4x + 3}\,\mathrm{d}x - \frac{1}{2} \int \frac{1}{4x^2 - 4x + 3}\,\mathrm{d}x
  • 第一項積分

    188x44x24x+3dx=18ln(4x24x+3)\frac{1}{8} \int \frac{8x - 4}{4x^2 - 4x + 3}\,\mathrm{d}x = \frac{1}{8}\ln(4x^2 - 4x + 3)
  • 第二項積分(將分母配方)

    4x24x+3=(2x1)2+2=(2x1)2+(2)24x^2 - 4x + 3 = (2x - 1)^2 + 2 = (2x - 1)^2 + (\sqrt{2})^2

    u=2x1    du=2dx    dx=12duu = 2x-1 \implies \mathrm{d}u = 2\,\mathrm{d}x \implies \mathrm{d}x = \frac{1}{2}\,\mathrm{d}u

    1214x24x+3dx=121u2+(2)212du=141u2+(2)2du=1412tan1(u2)=28tan1(2x12)\begin{align*} \frac{1}{2} \int \frac{1}{4x^2 - 4x + 3}\,\mathrm{d}x =&\, \frac{1}{2} \int \frac{1}{u^2 + (\sqrt{2})^2} \cdot \frac{1}{2}\,\mathrm{d}u = \frac{1}{4} \int \frac{1}{u^2 + (\sqrt{2})^2}\,\mathrm{d}u \\[4mm] =&\, \frac{1}{4} \cdot \frac{1}{\sqrt{2}} \tan^{-1}\left(\frac{u}{\sqrt{2}}\right) = \frac{\sqrt{2}}{8} \tan^{-1}\left(\frac{2x - 1}{\sqrt{2}}\right) \end{align*}

第三步:合併所有項

4x23x+24x24x+3dx=x+18ln(4x24x+3)28tan1(2x12)+C\int \frac{4x^2 - 3x + 2}{4x^2 - 4x + 3}\,\mathrm{d}x = \boxed{x + \frac{1}{8}\ln(4x^2 - 4x + 3) - \frac{\sqrt{2}}{8} \tan^{-1}\left(\frac{2x - 1}{\sqrt{2}}\right) + C}

解法二:變數代換配方法

思路

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  1. 由於分母 4x24x+3=(2x1)2+24x^2-4x+3 = (2x-1)^2+2 含有 (2x1)(2x-1) 的配方結構,我們可先進行線性代換:令 u=2x1u = 2x-1
  2. 藉由 x=u+12x = \frac{u+1}{2} 將分子與分母完全轉化為 uu 的代數式。
  3. 此時被積函數可分項化簡,避免在原自變數 xx 之下進行複雜的湊項計算。

答題過程

展開

u=2x1    x=u+12    dx=12duu = 2x - 1 \implies x = \frac{u+1}{2} \implies \mathrm{d}x = \frac{1}{2}\,\mathrm{d}u

將分子與分母以 uu 表達:

  • 分母: 4x24x+3=(2x1)2+2=u2+24x^2 - 4x + 3 = (2x-1)^2 + 2 = u^2 + 2

  • 分子:

    4x23x+2=4(u+12)23(u+12)+2=(u2+2u+1)32(u+1)+2=u2+12u+324x^2 - 3x + 2 = 4\left(\frac{u+1}{2}\right)^2 - 3\left(\frac{u+1}{2}\right) + 2 = (u^2 + 2u + 1) - \frac{3}{2}(u+1) + 2 = u^2 + \frac{1}{2}u + \frac{3}{2}

將變數代入積分式:

4x23x+24x24x+3dx=u2+12u+32u2+212du=12(u2+2)+12u12u2+2du=12(1+12uu2+2121u2+2)du=12u+18ln(u2+2)142tan1(u2)+C0\begin{align*} \int \frac{4x^2 - 3x + 2}{4x^2 - 4x + 3}\,\mathrm{d}x =&\, \int \frac{u^2 + \frac{1}{2}u + \frac{3}{2}}{u^2 + 2} \cdot \frac{1}{2}\,\mathrm{d}u \\[4mm] =&\, \frac{1}{2} \int \frac{(u^2+2) + \frac{1}{2}u - \frac{1}{2}}{u^2 + 2}\,\mathrm{d}u \\[4mm] =&\, \frac{1}{2} \int \left( 1 + \frac{1}{2}\frac{u}{u^2+2} - \frac{1}{2}\frac{1}{u^2+2} \right)\,\mathrm{d}u \\[4mm] =&\, \frac{1}{2}u + \frac{1}{8}\ln(u^2+2) - \frac{1}{4\sqrt{2}}\tan^{-1}\left(\frac{u}{\sqrt{2}}\right) + C_0 \end{align*}

u=2x1u = 2x-1 代回:

I=12(2x1)+18ln((2x1)2+2)28tan1(2x12)+C0=x12+18ln(4x24x+3)28tan1(2x12)+C0\begin{align*} I =&\, \frac{1}{2}(2x-1) + \frac{1}{8}\ln\left((2x-1)^2+2\right) - \frac{\sqrt{2}}{8}\tan^{-1}\left(\frac{2x-1}{\sqrt{2}}\right) + C_0 \\[4mm] =&\, x - \frac{1}{2} + \frac{1}{8}\ln(4x^2-4x+3) - \frac{\sqrt{2}}{8}\tan^{-1}\left(\frac{2x-1}{\sqrt{2}}\right) + C_0 \end{align*}

合併常數項 12+C0-\frac{1}{2} + C_0 為新的積分常數 CC

I=x+18ln(4x24x+3)28tan1(2x12)+CI = \boxed{x + \frac{1}{8}\ln(4x^2-4x+3) - \frac{\sqrt{2}}{8}\tan^{-1}\left(\frac{2x-1}{\sqrt{2}}\right) + C}