Answer
$R=k\left(\dfrac{L_{0}-s\cot\theta}{r_1^{4}}+\dfrac{s}{ r_2^{4}}\csc(\theta)\right)$
Work Step by Step
$$R=k\left(\dfrac{L_{1}}{r_1^{4}}+\dfrac{L_{2}}{r_2^{4}}\right)$$
Using the results of the pervious exercises it follows:
$$R=k\left(\dfrac{L_{0}-s\cot\theta}{r_1^{4}}+\dfrac{s}{\sin\theta r_2^{4}}\right)$$
$$R=k\left(\dfrac{L_{0}-s\cot\theta}{r_1^{4}}+\dfrac{s}{ r_2^{4}}\csc(\theta)\right)$$
