Q.From the equation \( \sin(90^\circ + \theta) = \cos(120^\circ - 3\theta) \), the value of \( \theta \) is — (a) 30° (b) 60° (c) 45° (d) None of the above
Answer: B
Given: \( \sin(90^\circ + \theta) = \cos(120^\circ - 3\theta) \) ⇒ \( \sin(90^\circ + \theta) = \sin[90^\circ - (120^\circ - 3\theta)] \) ⇒ \( 90^\circ + \theta = 90^\circ - 120^\circ + 3\theta \) ⇒ \( \theta - 3\theta = 90^\circ - 120^\circ - 90^\circ \) ⇒ \( -2\theta = -120^\circ \) ⇒ \( \theta = 60^\circ \)
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