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Can't Rearrange this equation ?

  • Thread starter skaboy607
  • Start date
  • #1
108
0
Can't Rearrange this equation!!!!!!???????

Homework Statement



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

Homework 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]

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:

Answers and Replies

  • #2
392
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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.
 
  • #3
108
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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
 
  • #4
392
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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.
 
  • #5
108
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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?
 
  • #6
392
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Now you've got it! It won't be long now!
 
  • #7
108
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I can't simplify that any further so shall I replace my values for a and c in now, then simplify?

Thanks
 
  • #8
392
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You've got fractions. Deal with that first.
 
  • #9
108
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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!
 
  • #10
392
0


Aha! Common denominator problem (or similar).

Early on, you should have written Y=cY/c.
 
  • #11
108
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erm..........i'm lost, where should that have been? Wouldn't the c's cancel to leave Y=Y anyway?
 
  • #12
392
0


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?
Look at right hand side. Get the common denominator here. Three terms have denominator c, but two don't.
 
  • #13
108
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Thanks very much for your help! Sorted it!
 

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