Confusing

Geometry Level 4

cos A csc B csc C + cos B csc A csc C + cos C csc A csc B \cos A \csc B \csc C + \cos B \csc A \csc C+\cos C \csc A \csc B

If A + B + C = π A+B+C=\pi , find the value of above expression.


The answer is 2.

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2 solutions

Rishabh Jain
Jun 16, 2016

Let the given expression be G \mathfrak G . I'll be using I . cos ( A + B ) = cos C ( By taking cos of A + B = π C ) \small{\color{#D61F06}{I. ~\cos (A+B)=-\cos C}(\text{By taking }\cos \text{ of } A+B=\pi-C)} I I . tan A + tan B + tan C = tan A tan B tan C ( By taking tan of A + B + C = π ) \small{\color{#D61F06}{II.~\tan A+\tan B+\tan C=\tan A\tan B\tan C}(\text{By taking }\tan \text{ of } A+B+C=\pi)} I I I . csc y = 1 sin y \small{\color{#D61F06}{III. \csc y=\dfrac1{\sin y}}} and I I I I . cos ( x + y ) = cos x cos y sin x sin y \small{\color{#D61F06}{IIII.~\cos (x+y)=\cos x\cos y-\sin x\sin y}} .

G = cyc ( cos ( B + C ) sin B sin C ) \large\mathfrak G=-\displaystyle\sum_{\text{cyc}}\left(\dfrac{\cos (B+C)}{\sin B \sin C}\right) = cyc ( 1 cot B cot C ) \large =\displaystyle\sum_{\text{cyc}}\left(1-\cot B\cot C\right) = 3 cyc ( tan A tan A tan B tan C ) 1 \large =3-\underbrace{\displaystyle\sum_{\text{cyc}}\left(\dfrac{\tan A}{\tan A\tan B\tan C}\right)}_{\large 1}

G = 2 \huge \therefore \mathfrak G=\boxed{\color{#007fff}{2}}


PS: Don't write in comments: just assume A = B = C = π 3 A=B=C=\dfrac{\pi}3 and you are done XD

That was really fast!

Akshat Sharda - 4 years, 12 months ago

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Lol ... Thanks... Now its already six months since I've started writing in latex :-D

Rishabh Jain - 4 years, 12 months ago

Or we could have used conditional trigonometric identities that if A+B+C =Pi

then sin2A+Sin2B+sin2C = 4sinAsinBsinC

Prakhar Bindal - 4 years, 12 months ago

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Yep... There could be so many ways to reach the final answer... :-)

Rishabh Jain - 4 years, 12 months ago

Why only π 3 \dfrac \pi 3 any A and B , A + B < π \pi and C= π \pi - A - B will be OK.

Niranjan Khanderia - 4 years, 12 months ago
Ahmad Saad
Jun 16, 2016

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