Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: How to solve for q after finding derivative (derived) equation?

  1. Sep 25, 2012 #1
    I have started with the formula.

    T = 0.5q+4q^(-1/2)+9/q

    I have derived and am trying to solve for q but have gotten caught up on how to solve when I have to subtract two variables with different base numbers and different exponents.

    2aj3l7q.jpg

    Any ideas?
     
    Last edited: Sep 25, 2012
  2. jcsd
  3. Sep 25, 2012 #2

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    For one thing you made a mistake to start. If you divide both sides by q then the left side should be T/q. The correct way to go is probably to set q^(-1/2)=w, then express the other powers of q in terms w. If you work it through you'll get a quartic equation in w, a polynomial equation with w^4 as the highest power. The good news is that in principle, has a analytic solution. The bad news is that it's so complex as to to be basically useless. Are you sure you haven't made a mistake in deriving the equation you showed us? Where did it come from??
     
  4. Sep 25, 2012 #3
    Sorry for the misunderstanding: The second line is actually the derivative in respect to q of the first line (I forgot to put ' after T). Then from the second line I want to find what q is (after setting T to 0).
     
  5. Sep 25, 2012 #4

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    Ah, ok. I should have spotted that. If you don't have a T in it then you can still change it into a quartic equation in w. Then you could hope it factors, but I don't think it does. If not you'll just have to find the roots numerically.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook