Trigonometric Functions · Superchallenge problem

Let $P=(\cos\theta,\sin\theta)$ be a point on the unit circle with $0<\theta<\dfrac{\pi}{2}$. The line tangent to the circle at $P$ meets the $x$-axis at $A$ and the $y$-axis at $B$. If $PA=3\cdot PB$, find $\theta$ in radians.