Ugly summation

Geometry Level 3

Evaluate [ 4 sin ( 11 12 π ) + 4 cos ( 23 12 π ) + 2 2 tan ( 19 12 π ) ] 2 \left[4\sin \left(\frac{11}{12}\pi\right) + 4\cos \left(\frac{23}{12}\pi\right) + 2\sqrt{2}\tan\left(\frac{19}{12}\pi\right)\right]^2 .


The answer is 32.

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1 solution

DreamRunner Moshi
Dec 21, 2013

My solution:

= 4 sin ( 11 12 π ) + 4 cos ( 23 12 π ) + 2 2 tan ( 19 12 π ) =4\sin (\frac{11}{12}\pi)+4\cos (\frac{23}{12}\pi)+2\sqrt{2}\tan (\frac{19}{12}\pi) = 4 sin ( π π 12 ) + 4 cos ( 2 π π 12 ) + 2 2 tan ( 2 π 5 π 12 ) =4\sin (\pi-\frac{\pi}{12})+4\cos (2\pi-\frac{\pi}{12})+2\sqrt{2}\tan (2\pi-\frac{5\pi}{12}) = 4 sin ( π 12 ) + 4 cos ( π 12 ) 2 2 tan ( 5 π 12 ) . . . . . . . . . ( i ) =4\sin (\frac{\pi}{12})+4\cos (\frac{\pi}{12})-2\sqrt{2}\tan (\frac{5\pi}{12}) ...\space ...\space ...(i)

Now sin θ 2 = 1 cos θ 2 \sin\frac{\theta}{2}=\sqrt{\frac{1-\cos\theta}{2}}

so sin π 12 = 1 cos 5 π 6 2 = 2 + 3 2 \sin\frac{\pi}{12}=\sqrt{\frac{1-\cos\frac{5\pi}{6}}{2}} = \frac{\sqrt{2+\sqrt{3}}}{2} we know sin θ = 1 cos 2 θ \sin\theta=\sqrt{1-\cos^{2}\theta} so cos π 12 = 2 3 2 \cos\frac{\pi}{12}=\frac{\sqrt{2-\sqrt{3}}}{2}

by putting these values in (i) , squaring and some calculations we have answer 32 \boxed{32}

Good job!

Victor Loh - 7 years, 4 months ago

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