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

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Solution

$c o s 3 A . (2 s i n 2 A - 1) = 0$
$c o s 3 A = 0$ or $2 s i n 2 A = 1$
$3 A = 90$ or $2 A = 30$
$A = 30^{\circ}$ or $A = 15^{\circ}$
Thus, $A = 30^{\circ}, 15^{\circ}$

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