Let $\theta$ (in radians) be an acute angle in a right triangle and let $x$ and $y$, respectively, be the lengths of the sides adjacent to and opposite $\theta$. Suppose also that $x$ and $y$ vary with time.
At a certain instant $x = 9$ units and is increasing at $8$ unit/s, while $y = 6$ and is decreasing at $\frac{1}{5}$ units/s.
How fast is $\theta$ changing at that instant?