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Calculus Level 5

0 π 4 tan x 1 tan x d x \large \int_{0}^{\frac{\pi}{4}}\sqrt{\tan x}\sqrt{1-\tan x }\ dx

If the value of above integral can be expressed as π ( cos ϕ 1 + cos ϕ 2 cos ϕ 3 ) \displaystyle \pi\left( \sqrt{\cos \phi_1 + \cos \phi_2 }- \cos \phi_3\right) , where ϕ 1 , ϕ 2 , ϕ 3 [ 0 , π ) \phi_1, \phi_2, \phi_3 \in\mathbb [0,\pi) . Find the value of ϕ 1 + ϕ 2 + ϕ 3 \phi_1+\phi_2+\phi_3 , and give your answer in degree.


The answer is 105.

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1 solution

Mark Hennings
Aug 8, 2018

The quadratic equation z 2 2 ( 1 + 2 i ) z + 1 = 0 z^2 - 2(1+2i)z + 1 = 0 has zeros λ ± = 1 + 2 i ± 2 5 4 e 3 π i 8 \lambda_\pm = 1 + 2i \pm 2^{\frac54}e^{\frac{3\pi i}{8}} , where λ + > 1 > λ |\lambda_+| > 1 > |\lambda_-| , and hence 0 2 π d θ cos θ 1 2 i = z = 1 2 z + z 1 2 ( 1 + 2 i ) d z i z = 2 i z = 1 d z z 2 2 ( 1 + 2 i ) z + 1 = 4 π λ λ + = π 2 1 4 e 3 π i 8 \int_0^{2\pi} \frac{d\theta}{\cos\theta - 1 - 2i} \; = \; \int_{|z|=1} \frac{2}{z+z^{-1}-2(1+2i)}\,\frac{dz}{iz} \; = \; \frac{2}{i}\int_{|z|=1}\frac{dz}{z^2 - 2(1+2i)z+1} \; = \; \frac{4\pi}{\lambda_- - \lambda_+} \; = \; -\frac{\pi}{2^{\frac14}}e^{-\frac{3\pi i}{8}} by standard Residue Calculus tricks. Thus, with the substitutions u = tan x u= \tan x , u = sin 2 θ u = \sin^2\theta , followed by a move to double angles, partial fractions and finally the above result, we obtain that 0 1 4 π tan x ( 1 tan x ) d x = 0 1 u ( 1 u ) 1 + u 2 d u = 0 1 2 π 2 sin 2 θ cos 2 θ 1 + sin 4 θ d θ = 2 0 1 2 π ( 1 + sin 2 θ 1 + sin 4 θ 1 ) d θ = 4 0 1 2 π 3 cos 2 θ 4 + ( 1 cos 2 θ ) 2 d θ π = 2 0 π 3 cos θ 4 + ( 1 cos θ ) 2 d θ π = 0 2 π 3 cos θ 4 + ( 1 cos θ ) 2 d θ π = 1 2 ( 1 + i ) 0 2 π d θ cos θ 1 2 i 1 2 ( 1 i ) 0 2 π d θ cos θ 1 + 2 i π = R e [ ( 1 + i ) 0 2 π d θ cos θ 1 2 i ] π = R e [ 2 e π i 4 π 2 1 4 e 3 π i 8 ] π = 2 1 4 π cos 1 8 π π = π ( 1 2 + 1 2 1 ) \begin{aligned} \int_0^{\frac14\pi} \sqrt{\tan x (1 - \tan x)}\,dx & = \; \int_0^1 \frac{\sqrt{u(1-u)}}{1+u^2}\,du \; = \; \int_0^{\frac12\pi} \frac{2\sin^2\theta \cos^2\theta}{1 + \sin^4\theta}\,d\theta \; = \; 2\int_0^{\frac12\pi}\left(\frac{1 + \sin^2\theta}{1 + \sin^4\theta} - 1\right)\,d\theta \\ & = \; 4\int_0^{\frac12\pi}\frac{3 - \cos2\theta}{4 + (1 - \cos2\theta)^2}\,d\theta - \pi \; = \; 2\int_0^\pi \frac{3 - \cos\theta}{4 + (1 - \cos\theta)^2}\,d\theta - \pi \\ & = \; \int_0^{2\pi} \frac{3 - \cos\theta}{4 + (1 - \cos\theta)^2}\,d\theta - \pi \; = \; -\tfrac12(1+i)\int_0^{2\pi} \frac{d\theta}{\cos\theta - 1 - 2i} - \tfrac12(1-i)\int_0^{2\pi} \frac{d\theta}{\cos\theta - 1 + 2i} - \pi \\ & = \; -\mathfrak{Re}\left[(1 + i)\int_0^{2\pi}\frac{d\theta}{\cos\theta - 1 - 2i}\right] - \pi \; = \; \mathfrak{Re}\left[ \sqrt{2}e^{\frac{\pi i}{4}} \frac{\pi}{2^{\frac14}}e^{-\frac{3\pi i}{8}}\right] - \pi \; = \; 2^{\frac14}\pi \cos \tfrac18\pi - \pi \\ & = \; \pi\left(\sqrt{\tfrac12 + \tfrac{1}{\sqrt{2}}} - 1\right) \end{aligned} which makes ϕ 1 + ϕ 2 + ϕ 3 = 60 + 45 + 0 = 105 \phi_1 + \phi_2 + \phi_3 = 60 + 45 + 0 = \boxed{105} .

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