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Rearranging terms in Trig equation

  1. Aug 20, 2011 #1
    I was reading on planetary motion and have gotten hung up on a "rearrangement of terms" that the author skimmed over. It reads that:


    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?

  2. jcsd
  3. Aug 20, 2011 #2
    Put all the terms including [itex]r[/itex] on the left hand side and then factor out the [itex]r[/itex]. 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
  4. Aug 20, 2011 #3


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    Try getting all the "r" terms onto one side of the equation first. That is, try [strike]adding[/strike] subtracting [itex]e\, r\, \cos(\theta)[/itex] to both sides.
    Last edited: Aug 20, 2011
  5. Aug 20, 2011 #4
    You mean subtracting, surely.
  6. Aug 20, 2011 #5


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    Um yeah. Add the negative. ;)

    BTW. We both posted at the same time. :)
  7. Aug 20, 2011 #6
    Thank you two, I guess it's time for me to go back and audit some pre-algebra classes... :)
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