Trigonometry!

Geometry Level 3

Find the sum of all values of α \alpha in the interval [ 0 , 36 0 ) [ 0^\circ, 360^\circ) for which

cos ( 2 α ) = 2 sin ( α ) cos ( α ) . \cos(2\alpha) = 2\sin(\alpha) \cos(\alpha) .


The answer is 630.

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2 solutions

Ron Gallagher
Jan 20, 2021

Since 2 sin(alpha) cos(alpha) = sin(2*alpha), we can rewrite the equation:

cos(2 alpha) = sin(2 alpha), or, dividing:

tan(2*alpha) = 1.

There are solutions where the tangent is positive (in the first and third quadrant):

2 alpha = 45 + 180 n (n can be any integer) or

2 alpha = 225 + 180 n (n can be any integer).

Dividing by two shows all solutions are of the form:

alpha = 22.5 + 90*n or

alpha = 112.5 + 90*n (n can be any integer).

Since we are restricted to the given interval, we find:

alpha = 22.5, 112.5, 202.5, or 292.5

Summing these four possibilities gives 630

Chew-Seong Cheong
Jan 21, 2021

cos 2 α = 2 sin α cos α = sin 2 α tan 2 α = 1 2 α = 4 5 , 22 5 , 40 5 , 58 5 α = 22. 5 , 112. 5 , 202. 5 , 292. 5 \begin{aligned} \cos 2\alpha & = 2 \sin \alpha \cos \alpha = \sin 2 \alpha \\ \tan 2 \alpha & = 1 \\ 2 \alpha & = 45^\circ, 225^\circ, 405^\circ, 585^\circ \\ \implies \alpha & = 22.5^\circ, 112.5^\circ, 202.5^\circ, 292.5^\circ \end{aligned}

Therefore the answer is 22.5 + 112.5 + 202.5 + 292.5 = 630 22.5 + 112.5 + 202.5 + 292.5 = \boxed{630} .

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