A curve GG has equation y=1cos⁡x+cos⁡xy = \frac{1}{\cos x} + \cos x. The region bounded by GG, the line x=π3x = \frac{\pi}{3}, the xx-axis and the yy-axis is rotated through 4 right angles about the xx-axis. Find the exact volume generated, giving your answer in terms of π\pi.

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Worked solution

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1. Set up the definite integral for the volume of revolution.

V=π∫0π3y2 dxV = \pi \int_{0}^{\frac{\pi}{3}} y^2 \, dx
V=π∫0π3(1cos⁡x+cos⁡x)2 dxV = \pi \int_{0}^{\frac{\pi}{3}} \left( \frac{1}{\cos x} + \cos x \right)^2 \, dx

2. Expand the squared expression and apply trigonometric identities.

(sec⁡x+cos⁡x)2=sec⁡2x+2sec⁡xcos⁡x+cos⁡2x\left( \sec x + \cos x \right)^2 = \sec^2 x + 2\sec x \cos x + \cos^2 x
=sec⁡2x+2+cos⁡2x= \sec^2 x + 2 + \cos^2 x
Using the double angle formula cos⁡2x=12+12cos⁡2x\cos^2 x = \frac{1}{2} + \frac{1}{2}\cos 2x:
V=π∫0π3(sec⁡2x+52+12cos⁡2x) dxV = \pi \int_{0}^{\frac{\pi}{3}} \left( \sec^2 x + \frac{5}{2} + \frac{1}{2}\cos 2x \right) \, dx

3. Integrate the expression with respect to $x$.

∫(sec⁡2x+52+12cos⁡2x) dx=tan⁡x+52x+14sin⁡2x\int \left( \sec^2 x + \frac{5}{2} + \frac{1}{2}\cos 2x \right) \, dx = \tan x + \frac{5}{2}x + \frac{1}{4}\sin 2x

4. Evaluate the definite integral using exact trigonometric values.

Evaluate from x=0x = 0 to x=π3x = \frac{\pi}{3}:
At x=π3x = \frac{\pi}{3}:
tan⁡(π3)+52(π3)+14sin⁡(2π3)=3+5π6+14(32)\tan\left(\frac{\pi}{3}\right) + \frac{5}{2}\left(\frac{\pi}{3}\right) + \frac{1}{4}\sin\left(\frac{2\pi}{3}\right) = \sqrt{3} + \frac{5\pi}{6} + \frac{1}{4}\left(\frac{\sqrt{3}}{2}\right)
=3+5π6+38=938+5π6= \sqrt{3} + \frac{5\pi}{6} + \frac{\sqrt{3}}{8} = \frac{9\sqrt{3}}{8} + \frac{5\pi}{6}
At x=0x = 0:
tan⁡(0)+52(0)+14sin⁡(0)=0\tan(0) + \frac{5}{2}(0) + \frac{1}{4}\sin(0) = 0
V=π(938+5π6)−0=π(938+5π6)V = \pi \left( \frac{9\sqrt{3}}{8} + \frac{5\pi}{6} \right) - 0 = \pi \left( \frac{9\sqrt{3}}{8} + \frac{5\pi}{6} \right)

Answer

π(938+5π6)\pi \left( \frac{9\sqrt{3}}{8} + \frac{5\pi}{6} \right)

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