Find the other value of A, if $cos 3 A (2 sin 2 A - 1) = 0$ and one value of A is $30^{o}$ .

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Solution

$cos 3 A (2 sin 2 A - 1) = 0$
Either, $cos 3 A = 0$ or $2 sin 2 A - 1 = 0$
$cos 3 A = cos 90$ or $sin 2 A = \frac{1}{2}$
$3 A = 90$ or $sin 2 A = sin 30$
$A = 30^{\circ}$ or $2 A = 30$
$A = 30^{\circ}$ or $A = 15^{\circ}$

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