Evaluating Sine

Calculus Level 2

sin ( 1 ) sin ( 2 ) sin ( 3 ) sin ( 8 9 ) cos ( 1 ) + cos ( 2 ) + cos ( 3 ) + + cos ( 8 9 ) = ? \frac {-\sin \left(-1^\circ\right) -\sin \left(-2^\circ\right) -\sin \left(-3^\circ\right)-\cdots -\sin \left(-89^\circ\right)}{\cos \left(-1^\circ\right) +\cos \left(-2^\circ\right) + \cos \left(-3^\circ\right) + \cdots + \cos \left(-89^\circ\right)} = \ ?

1 2 4 3

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2 solutions

Chew-Seong Cheong
Apr 21, 2017

Q = sin ( 1 ) sin ( 2 ) sin ( 3 ) sin ( 8 9 ) cos ( 1 ) + cos ( 2 ) + cos ( 3 ) + + cos ( 8 9 ) Note that sin ( θ ) = sin θ , cos ( θ ) = cos θ = sin ( 1 ) + sin ( 2 ) + sin ( 3 ) + + sin ( 8 9 ) cos ( 1 ) + cos ( 2 ) + cos ( 3 ) + + cos ( 8 9 ) Note that cos ( 9 0 θ ) = sin θ = sin ( 1 ) + sin ( 2 ) + sin ( 3 ) + + sin ( 8 9 ) sin ( 8 9 ) + sin ( 8 8 ) + sin ( 8 7 ) + + sin ( 1 ) = 1 \begin{aligned} Q & = \frac {-\sin \left(-1^\circ\right) -\sin \left(-2^\circ\right) -\sin \left(-3^\circ\right)-\cdots -\sin \left(-89^\circ\right)}{\cos \left(-1^\circ\right) +\cos \left(-2^\circ\right) + \cos \left(-3^\circ\right) + \cdots + \cos \left(-89^\circ\right)} & \small \color{#3D99F6} \text{Note that }\sin (-\theta) = - \sin \theta, \ \cos(-\theta) = \cos \theta \\ & = \frac {\sin \left(1^\circ\right) + \sin \left(2^\circ\right) + \sin \left(3^\circ\right)+\cdots +\sin \left(89^\circ\right)}{\cos \left(1^\circ\right) +\cos \left(2^\circ\right) + \cos \left(3^\circ\right) + \cdots + \cos \left(89^\circ\right)} & \small \color{#3D99F6} \text{Note that }\cos (90^\circ-\theta) = \sin \theta \\ & = \frac {\sin \left(1^\circ\right) + \sin \left(2^\circ\right) + \sin \left(3^\circ\right)+\cdots +\sin \left(89^\circ\right)}{\sin \left(89^\circ\right) + \sin \left(88^\circ\right) + \sin \left(87^\circ\right)+\cdots +\sin \left(1^\circ\right)} \\ & = \boxed{1} \end{aligned}

Danilo Fronda
Apr 20, 2017

This is the same as Summation of sin x from 1 to 89 all over summation of cos x from 1 to 89 degrees. Take note that by co-function, sin 1 = cos 89. Thus the numerator is the same as denominstor Therefore, the answer is 1.

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