Find the sum of all values of α in the interval [ 0 ∘ , 3 6 0 ∘ ) for which
cos ( 2 α ) = 2 sin ( α ) cos ( α ) .
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cos 2 α tan 2 α 2 α ⟹ α = 2 sin α cos α = sin 2 α = 1 = 4 5 ∘ , 2 2 5 ∘ , 4 0 5 ∘ , 5 8 5 ∘ = 2 2 . 5 ∘ , 1 1 2 . 5 ∘ , 2 0 2 . 5 ∘ , 2 9 2 . 5 ∘
Therefore the answer is 2 2 . 5 + 1 1 2 . 5 + 2 0 2 . 5 + 2 9 2 . 5 = 6 3 0 .
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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