Solve:
$cos 3 A . (2 sin 2 A - 1) = 0$

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Solution

$cos 3 A (2 sin 2 A - 1) = 0$
$(sin 2 A = \frac{1}{2} \to 2 A = \frac{π}{6} + 2 k π o r \frac{5 π}{6} + 2 k π)$
$\Rightarrow cos 3 A = 0$
$\Rightarrow 3 A = \frac{π}{2} (2 k + 1)$
Hence, solved.

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