2. (C) One way to estimate moment arm of a muscle with respect to a particular degree of freedom is to measure the length of the muscle-tendon complex over a range of joint angles and then use the following equation, where $r$ = moment arm, $l^{MT}$ is the length of the muscle-tendon complex, and $\theta$ is the joint angle.
$r = \frac{\delta l^{MT}}{\delta \theta}$
Use the figure below, along with the law of sines and the law of cosines, to prove the above equation.
[Diagram showing a triangle with sides a, b, and hypotenuse $l^{MT}$. A perpendicular line of length r is drawn from the hypotenuse to the opposite vertex. Angles $\theta$ and $\phi$ are labeled.]
