#2015_21 KVPY 2015 preparations!

Geometry Level 3

64 sin θ + 64 cos θ 8 sec θ 8 csc θ + tan θ + cot θ . \large 64\sin { \theta } +64\cos { \theta } -8\sec { \theta } -8\csc { \theta } +\tan { \theta } +\cot { \theta } .

1f θ = 1 2 sin 1 ( 1 4 ) \theta =\frac 12 \sin ^{ -1 }\left(\frac 14\right) , find the value of the expression above.


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The answer is 8.

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2 solutions

Vishnu Bhagyanath
Oct 29, 2015

From the given condition, we can say 2 s i n θ c o s θ = 1 4 2 sin \theta cos \theta = \frac14 .

Simplifying the given equation, 64 ( s i n θ + c o s θ ) 8 ( s i n θ + c o s θ ) 1 8 + s i n 2 θ + c o s 2 θ 1 8 64(sin \theta + cos \theta) - 8 \frac{(sin \theta + cos \theta )}{\frac{1}{8}}+ \frac{sin^2 \theta + cos^2 \theta}{\frac18} 64 ( s i n θ + c o s θ ) 64 ( s i n θ + c o s θ ) + 8 64(sin \theta + cos \theta) - 64(sin \theta + cos \theta) +8

From what is given:

θ = 1 2 sin 1 1 4 2 θ = sin 1 1 4 sin 2 θ = 1 4 2 sin θ cos θ = 1 4 sin θ cos θ = 1 8 \begin{aligned} \theta & = \frac 12 \sin^{-1} \frac 14 \\ 2 \theta & = \sin^{-1} \frac 14 \\ \sin 2 \theta & = \frac 14 \\ 2\sin \theta \cos \theta & = \frac 14 \\ \sin \theta \cos \theta & = \frac 18 \end{aligned}

Then we have:

x = 64 sin θ + 64 cos θ 8 sec θ 8 csc θ + tan θ + cot θ = 64 sin θ + 64 cos θ 8 cos θ 8 sin θ + sin θ cos θ + cos θ sin θ = 64 sin 2 θ cos θ + 64 sin θ cos 2 θ 8 sin θ 8 cos θ + sin 2 θ + cos 2 θ sin θ cos θ Note that sin θ cos θ = 1 8 = 8 sin θ + 8 cos θ 8 sin θ 8 cos θ + 1 1 8 = 8 \begin{aligned} x & = 64\sin \theta + 64 \cos \theta - 8 \sec \theta - 8 \csc \theta + \tan \theta + \cot \theta \\ & = 64\sin \theta + 64 \cos \theta - \frac 8{\cos \theta} -\frac 8{\sin \theta} + \frac {\sin \theta}{\cos \theta} + \frac {\cos \theta}{\sin \theta} \\ & = \frac {64\sin^2 \theta \cos \theta + 64 \sin \theta \cos^2 \theta - 8 \sin \theta - 8 \cos \theta + \sin^2 \theta + \cos^2 \theta}{\sin \theta \cos \theta} & \small \color{#3D99F6} \text{Note that }\sin \theta \cos \theta = \frac 18 \\ & = \frac {8\sin \theta + 8 \cos \theta - 8 \sin \theta - 8 \cos \theta + 1}{\frac 18} \\ & = \boxed{8} \end{aligned}

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