Porisma

Worked solution

Split a numerator into a denominator derivative and a remainder

Express x+3=A(2x+2)+Bx+3=A(2x+2)+B, where AA and BB are constants to be determined.

Hence, find ∫x+3x2+2x+2 dx\displaystyle\int\dfrac{x+3}{x^2+2x+2}\,dx. marks

[2019/RVHS/Promo/4(a)]

CJC · Review Problems · Question 12

Answer

A=1/2A=1/2, B=2B=2; 12ln⁡∣x2+2x+2∣+2tan⁡−1(x+1)+C\frac12\ln|x^2+2x+2|+2\tan^{-1}(x+1)+C.

Constants

Compare coefficients: A=1/2A=1/2, and 2A+B=32A+B=3 gives B=2B=2.

How did this part go?

Hence integral

Split the numerator to get 12∫2x+2x2+2x+2dx+2∫11+(x+1)2dx\frac12\int\dfrac{2x+2}{x^2+2x+2}dx+2\int\dfrac1{1+(x+1)^2}dx. This is 12ln⁡∣x2+2x+2∣+2tan⁡−1(x+1)+C\frac12\ln|x^2+2x+2|+2\tan^{-1}(x+1)+C. The quadratic is always positive, so the bars are optional.

How did this part go?

Common mistakes

The single three-mark allocation covers the whole printed question; do not invent a split.

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