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1 Pure Mathematics

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Question 83

A precision-engineered acoustic horn is modeled as a solid of revolution. The internal profile of the horn is defined by the curve C C\,C with equation

y=2x+xxarctan⁡(x) y = \frac{2}{\sqrt{\sqrt{x} + x\sqrt{x}} \arctan(\sqrt{x})} y=x​+xx​​arctan(x​)2​

for 13≤x≤1\frac{1}{3} \le x \le 131​≤x≤1. The region RRR is bounded by the curve CCC, the xxx-axis, and the vertical lines x=13x = \frac{1}{3}x=31​ and x=1x = 1x=1.

The internal volume V V\,V of the horn is formed by rotating the region RRR through 360∘ 360^\circ\,360∘ about the xxx-axis.

Using the substitution tan⁡u=x\tan u = \sqrt{x}tanu=x​,

a.

show that the volume VVV is given by

k∫ab1u2 du k \int_{a}^{b} \frac{1}{u^2} \, du k∫ab​u21​du

where k,ak, ak,a and bbb are constants to be found.

[5]
b.

Hence, using algebraic integration, find the exact value of VVV.

[3]

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