Solving a Rotating Platform Ride Problem

[jnthn205]

Homework Statement

An amusement park ride consists of a rotating circular platform 9.87m in diamter from which 10kg seats are suspended at the end of 2.59m massless chains (L). When the system rotates, the chains make an angle of 39.3 degrees with the vertical. The acceleration of gravity is 9.8m/s^2. What is the speed of each seat? Answer in m/s.

Homework Equations

Force = Mv^2/r

The Attempt at a Solution

look at this thread which is the exact same problem:
https://www.physicsforums.com/showthread.php?t=191527

I figured out how to do it from there. My question is how did that user get 6.6 for his radius and also how did he get his answer? I tried doing what he did, (even put his numbers in) and i still got different answers.

I found that my radius of the system = 7.525m correct?
And so I used sqrt((r + L*sin(theta)) *g*tan(theta))
which comes out to sqrt((7.525 + 2.59*sin(39.3)) *9.8*tan(39.3)).
Which is 8.57m/s. Before i checked my answer, i used the users numbers to find the answer, and it was different then what he said was correct. I think my formula is wrong, but I cannot figure out why.

 

[nvn]

jnthn205: Show your work for how you computed radius r. Your current radius is incorrect, but we do not yet know why, because you did not show how you computed r.