# Rearranging terms in Trig equation

1. Aug 20, 2011

### PTX-14

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!

2. Aug 20, 2011

### Nylex

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

3. Aug 20, 2011

### 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.

Last edited: Aug 20, 2011
4. Aug 20, 2011

### Nylex

You mean subtracting, surely.

5. Aug 20, 2011

### uart

Um yeah. Add the negative. ;)

BTW. We both posted at the same time. :)

6. Aug 20, 2011

### PTX-14

Thank you two, I guess it's time for me to go back and audit some pre-algebra classes... :)