Explanation:
The Law of Cosines can be used to write a quadratic in x. The desired form corresponds to its solution using the quadratic formula.
[tex]L^2=r^2 +x^2 -2rx\cdot\cos{\theta}\\\\x^2-2rx\cdot\cos{\theta}+(r^2-L^2)=0[/tex]
Using the quadratic formula gives the positive value of x as ...
[tex]x=\dfrac{-(-2r\cdot\cos{\theta})+\sqrt{(-2r\cdot\cos{\theta})^2-4(1)(r^2-L^2)}}{2(1)}\\\\\boxed{x=r\cos{\theta}+\sqrt{r^2\cos^2\theta+L^2-r^2}}[/tex]
