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**1. Homework Statement**

I'm trying to solve the 2-body problem analytically by following this book:

http://books.google.com/books?id=imN_8IuZ8l8C&lpg=PR1&dq=David Betounes Differential equations&pg=PA60#v=onepage&q&f=false"

(note: the book preview is not complete, but you can find pages 69-73 http://books.google.com/books?id=oNvFAzQXBhsC&lpg=PA58&pg=PA64#v=onepage&q&f=false", it's a different version of the book but this section is identical as far as i noticed)

On page 68, the last equation (2.35) gives me the solution to r, the magnitude of the displacement vector between the bodies. But how do I get the angle [tex]\theta[/tex]? It's a function of time too, but all the equations with it contain also r, which I don't have yet computed.

I understand how to get the constants (probably) and how to convert between the various coordinate systems, but I just don't see a way to get the angle [tex]\theta[/tex]. As far as I see it, r(t) depends on [tex]\theta[/tex](t) and [tex]\theta[/tex](t) depends on r(t), so I'm kind of stuck...

**2. Homework Equations**

Equation 2.35 in the first book link and 2.36 in the second.

**3. The Attempt at a Solution**

I wondered if maybe [tex]\theta'[/tex] is constant but it doesn't seem so. I think I am missing some basic concept :(

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