Porisma

Worked solution

Choose among inner derivatives, completed squares and parts

Find the following integrals:

(a) ∫e2xe2x−3 dx\displaystyle\int\dfrac{e^{2x}}{e^{2x}-3}\,dx marks

(b) ∫3x2+2x3+2x−8 dx\displaystyle\int\dfrac{3x^2+2}{\sqrt{x^3+2x-8}}\,dx marks

(c) ∫1x2+2x+5 dx\displaystyle\int\dfrac1{x^2+2x+5}\,dx marks

(d) ∫x2ln⁡x dx\displaystyle\int x^2\ln x\,dx marks

(e) ∫xsec⁡(x2)tan⁡(x2) dx\displaystyle\int x\sec(x^2)\tan(x^2)\,dx marks

[2018/MJC/Promo/2(modified)]

CJC · Review Problems · Question 11

Answer

(a) 12ln⁡∣e2x−3∣+C\frac12\ln|e^{2x}-3|+C; (b) 2x3+2x−8+C2\sqrt{x^3+2x-8}+C; (c) 12tan⁡−1((x+1)/2)+C\frac12\tan^{-1}((x+1)/2)+C; (d) 13x3ln⁡x−19x3+C\frac13x^3\ln x-\frac19x^3+C; (e) 12sec⁡(x2)+C\frac12\sec(x^2)+C.

(a)

Insert the factor 2 from the denominator derivative: 12∫2e2xe2x−3dx=12ln⁡∣e2x−3∣+C\frac12\int\dfrac{2e^{2x}}{e^{2x}-3}dx=\frac12\ln|e^{2x}-3|+C.

How did this part go?

(b)

The numerator is the derivative of x3+2x−8x^3+2x-8. Integrating (3x2+2)(x3+2x−8)−1/2(3x^2+2)(x^3+2x-8)^{-1/2} gives 2x3+2x−8+C2\sqrt{x^3+2x-8}+C on the real domain with positive radicand.

How did this part go?

(c)

Complete the square: x2+2x+5=(x+1)2+22x^2+2x+5=(x+1)^2+2^2. The integral is 12tan⁡−1((x+1)/2)+C\frac12\tan^{-1}((x+1)/2)+C.

How did this part go?

(d)

Integrate by parts with u=ln⁡xu=\ln x, dv=x2dxdv=x^2dx. Then v=x3/3v=x^3/3 and the answer is 13x3ln⁡x−13∫x2dx=13x3ln⁡x−19x3+C\frac13x^3\ln x-\frac13\int x^2dx=\frac13x^3\ln x-\frac19x^3+C.

How did this part go?

(e)

Insert the inner derivative 2x: 12∫(2x)sec⁡(x2)tan⁡(x2)dx=12sec⁡(x2)+C\frac12\int(2x)\sec(x^2)\tan(x^2)dx=\frac12\sec(x^2)+C.

How did this part go?

Common mistakes

Each antiderivative retains its original real domain and excluded denominator zeros.

Read the hint