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Mathematics · Ch 10 — Indefinite Integration

Integrals of the Form $\int\dfrac{px+q}{ax^2+bx+c}\,dx$ and $\int\dfrac{px+q}{\sqrt{ax^2+bx+c}}\,dx$

10.2.7

Integrals of the Form $\int\dfrac{px+q}{ax^2+bx+c}\,dx$ and $\int\dfrac{px+q}{\sqrt{ax^2+bx+c}}\,dx$

When the numerator is linear, write px+q=A⋅ddx(ax2+bx+c)+Bpx+q=A\cdot\dfrac{d}{dx}(ax^2+bx+c)+B for constants A,BA,B found by comparing coefficients. This splits ∫px+qax2+bx+c dx\displaystyle\int\dfrac{px+q}{ax^2+bx+c}\,dx into A∫ddx(ax2+bx+c)ax2+bx+c dx+B∫dxax2+bx+cA\displaystyle\int\dfrac{\frac{d}{dx}(ax^2+bx+c)}{ax^2+bx+c}\,dx+B\displaystyle\int\dfrac{dx}{ax^2+bx+c} — the first piece is a Corollary-III log (put ax2+bx+c=tax^2+bx+c=t), the second is the 3.2.4 type. Exactly the same split works for ∫px+qax2+bx+c dx\displaystyle\int\dfrac{px+q}{\sqrt{ax^2+bx+c}}\,dx, with the first piece becoming a Corollary-IV square root instead of a log.

∫2x−33x2+4x+5 dx\int\dfrac{2x-3}{3x^2+4x+5}\,dx. Write 2x−3=A(6x+4)+B2x-3=A(6x+4)+B; comparing coefficients, 6A=2⇒A=136A=2\Rightarrow A=\frac13, and 4A+B=−3⇒B=−1334A+B=-3\Rightarrow B=-\frac{13}3. So I=13∫6x+43x2+4x+5dx−133∫dx3x2+4x+5=13log⁡(3x2+4x+5)−13311tan⁡−13x+211+cI=\frac13\int\dfrac{6x+4}{3x^2+4x+5}dx-\frac{13}3\int\dfrac{dx}{3x^2+4x+5}=\frac13\log(3x^2+4x+5)-\dfrac{13}{3\sqrt{11}}\tan^{-1}\dfrac{3x+2}{\sqrt{11}}+c (the second integral evaluated by completing the square, 3.2.4-style).

∫x−5x−7 dx\int\sqrt{\dfrac{x-5}{x-7}}\,dx. Multiply inside the root by x−5x−5\frac{x-5}{x-5}: the integrand becomes x−5(x−5)(x−7)=x−5x2−12x+35\dfrac{x-5}{\sqrt{(x-5)(x-7)}}=\dfrac{x-5}{\sqrt{x^2-12x+35}}. Write x−5=12(2x−12)+1x-5=\frac12(2x-12)+1: this splits into 12∫2x−12x2−12x+35 dx+∫dxx2−12x+35\frac12\int\dfrac{2x-12}{\sqrt{x^2-12x+35}}\,dx+\int\dfrac{dx}{\sqrt{x^2-12x+35}}. The first is a Corollary-IV square root, x2−12x+35\sqrt{x^2-12x+35}; the second, after completing the square to (x−6)2−1(x-6)^2-1, is a Formula-5 log. Result: I=x2−12x+35+log⁡∣(x−6)+x2−12x+35∣+cI=\sqrt{x^2-12x+35}+\log\left|(x-6)+\sqrt{x^2-12x+35}\right|+c. …