Although I can't, as yet, help you in answering/checking questions as I am yet to go through the complete syllabus yet. However, I was wondering if you could help me with this questions:

Intro to Mechanics: Q.K. Ghori

Question 4

I can get to the first part of the result that is:

4r cos2(Theta) = L cos(theta)

However, it is the second part i.e. L > {(2(6)^0.5 . r) / 3

I look forward to your response.