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Worked solution

Scale difference-of-squares logarithmic forms

Integrate with respect to xx: (a) 1/(x2−4)1/(x^2-4); (b) 1/(4x2−1)1/(4x^2-1); (c) 1/(16−4x2)1/(16-4x^2).

CJC · Lecture · Example 7

(a)

Write 4=224=2^2. Then ∫1x2−4 dx=12(2)ln⁡∣x−2x+2∣+C=14ln⁡∣x−2x+2∣+C\int\frac1{x^2-4}\,dx=\frac1{2(2)}\ln\left|\frac{x-2}{x+2}\right|+C=\frac14\ln\left|\frac{x-2}{x+2}\right|+C, for x≠±2x\ne\pm2.

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(b) Printed method: factor 4

∫14x2−1 dx=14∫1x2−(1/2)2 dx=1412(1/2)ln⁡∣x−1/2x+1/2∣+C=14ln⁡∣2x−12x+1∣+C\int\frac1{4x^2-1}\,dx=\frac14\int\frac1{x^2-(1/2)^2}\,dx=\frac14\frac1{2(1/2)}\ln\left|\frac{x-1/2}{x+1/2}\right|+C=\frac14\ln\left|\frac{2x-1}{2x+1}\right|+C, for x≠±1/2x\ne\pm1/2.

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(b) Teacher alternative: composite form

The handwritten alternative takes f(x)=2xf(x)=2x and f′(x)=2f^{\prime}(x)=2: ∫1(2x)2−1 dx=12∫2(2x)2−1 dx=1212ln⁡∣2x−12x+1∣+C\int\frac1{(2x)^2-1}\,dx=\frac12\int\frac2{(2x)^2-1}\,dx=\frac12\frac12\ln\left|\frac{2x-1}{2x+1}\right|+C. This gives the same quarter-logarithm.

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(c)

∫116−4x2 dx=14∫122−x2 dx=1412(2)ln⁡∣2+x2−x∣+C=116ln⁡∣2+x2−x∣+C\int\frac1{16-4x^2}\,dx=\frac14\int\frac1{2^2-x^2}\,dx=\frac14\frac1{2(2)}\ln\left|\frac{2+x}{2-x}\right|+C=\frac1{16}\ln\left|\frac{2+x}{2-x}\right|+C, for x≠±2x\ne\pm2.

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Common mistakes

These rational denominators have no square root. Keep the original denominator zeros excluded and compensate for any scale factor.

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