# Rearranging terms in Trig equation

## Main Question or Discussion Point

I was reading on planetary motion and have gotten hung up on a "rearrangement of terms" that the author skimmed over. It reads that:

r=e(k+rcos(θ))=(ek)/(1-ecos(θ))

It's been a while since I've been in a math class: I just can't follow how to get from a to b. Is there anyone who can walk me through this like I'm twelve?

Thanks!

Put all the terms including $r$ on the left hand side and then factor out the $r$. It might help you to multiply out the brackets on the right hand side first:

r = e\left(k + r\cos \theta\right) = ek + er\cos \theta

uart
Try getting all the "r" terms onto one side of the equation first. That is, try [strike]adding[/strike] subtracting $e\, r\, \cos(\theta)$ to both sides.

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That is, try adding $r\, e\, \cos(\theta)$ to both sides.
You mean subtracting, surely.

uart