An object with weight W is dragged along a horizontal plane by a force acting along a rope attached to the object. If the rope makes an angle $\theta$ with the plane, then the magnitude of the force is

$F = \dfrac {\mu W} {\mu \sin ( \theta ) + \cos ( \theta ) }$

Where $\mu$ is a positive constant called the coefficient of friction and where $0 \leq \theta \leq \pi/2$. Find the value for $\tan \theta$ which minimizes the force. Your answer may depend on W and $\mu$, but use "u" instead of $\mu$ when entering your answer.
$\tan \theta =$