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Can't Rearrange this equation ?

  1. Apr 9, 2009 #1
    Can't Rearrange this equation!!!!!!???????

    1. The problem statement, all variables and given/known data

    I need to substitute one equation into another and rearrange for one term. I can't get the right answer!

    2. Relevant equations

    I've had to attach the equations as pictures im afraid, here they are.

    The equation to be substituted-http://i423.photobucket.com/albums/pp315/skaboy607/Tobesubstituted.png [Broken]

    Main Equation-http://i423.photobucket.com/albums/pp315/skaboy607/MainEquation.png [Broken]

    What I cant get!-http://i423.photobucket.com/albums/pp315/skaboy607/FinalEqthatIcantget.png [Broken]

    3. The attempt at a solution

    I have a few extra terms when I rearrange, it is driving me mad!

    Thanks for your help.
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 9, 2009 #2
    Re: Can't Rearrange this equation!!!!!!???????

    Make your life easier by writing a=ms^2+Bs+K and c=Bs+K. Also write X instead of X(s) and Y instead of Y(s). Note X=aY/c. Simplify as much as you can. Then substitute a=ms^2+c and simplify some more. It works out.
     
  4. Apr 9, 2009 #3
    Re: Can't Rearrange this equation!!!!!!???????

    ok, so far I using your easier notation I have U=cX-cY+Ms^2X, then with substitution, U=caY/c-cY+Ms^2aY/c.

    Is this what you meant? Im confused now with when to put a=ms^2+c.

    Thanks
     
  5. Apr 9, 2009 #4
    Re: Can't Rearrange this equation!!!!!!???????

    You're doing it backwards from the way I did it.

    First I rewrote "main equation" as U(s) = etc. Call this "new main equation."

    Then I used the easier notation in "tobesubstituted", then substituted into "new main equation." Then I simplified, etc.

    From time to time I looked at the goal "FinalEqthatIcantget" but I didn't start with it.
     
  6. Apr 9, 2009 #5
    Re: Can't Rearrange this equation!!!!!!???????

    Ok so starting from scratch, my rearranged 'main equation' looks like U=Ms^2X+kX-kY+bsX-bsY. Then Substituting in the easier notationed 'tobesubstitued' I have:

    U=Ms^2aY/c+kaY/c-ky+bsaY/c-bsY.

    This along the right lines?
     
  7. Apr 9, 2009 #6
    Re: Can't Rearrange this equation!!!!!!???????

    Now you've got it! It won't be long now!
     
  8. Apr 9, 2009 #7
    Re: Can't Rearrange this equation!!!!!!???????

    I can't simplify that any further so shall I replace my values for a and c in now, then simplify?

    Thanks
     
  9. Apr 9, 2009 #8
    Re: Can't Rearrange this equation!!!!!!???????

    You've got fractions. Deal with that first.
     
  10. Apr 9, 2009 #9
    Re: Can't Rearrange this equation!!!!!!???????

    ok.....do you mean multiply both sides by c to cancel it out? I tried without getting rid of the fractions, and was stuck with the same terms again as before that I cant get rid of. This are '2kbs', '(bs)^2', 'k' and 'k^2'. If it helps this is the equation where I am at.

    U=(Mms^4+Mbs^3+Mks^2+kms^2+kbs+k^2-k+mbs^3+(bs)^2+kbs-bs)Y/c

    Sorry if im being stupid!
     
  11. Apr 9, 2009 #10
    Re: Can't Rearrange this equation!!!!!!???????

    Aha! Common denominator problem (or similar).

    Early on, you should have written Y=cY/c.
     
  12. Apr 9, 2009 #11
    Re: Can't Rearrange this equation!!!!!!???????

    erm..........i'm lost, where should that have been? Wouldn't the c's cancel to leave Y=Y anyway?
     
  13. Apr 9, 2009 #12
    Re: Can't Rearrange this equation!!!!!!???????

    Look at right hand side. Get the common denominator here. Three terms have denominator c, but two don't.
     
  14. Apr 9, 2009 #13
    Re: Can't Rearrange this equation!!!!!!???????

    Thanks very much for your help! Sorted it!
     
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