JEE Advance - Mathematics (2000 - No. 22)
Let $$f\left( \theta \right) = \sin \theta \left( {\sin \theta + \sin 3\theta } \right)$$. Then $$f\left( \theta \right)$$ is
$$ \ge 0\,\,$$ only when $$\theta \ge 0$$
$$ \le 0$$ for all real $$\theta $$
$$ \ge 0$$ for all real $$\theta $$
$$ \le 0$$ only when $$\theta \le 0$$