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Homework Help: Simple Algebra problem that is killing me

  1. Feb 17, 2010 #1
    Simple Algebra problem that is killing me!!

    1. The problem statement, all variables and given/known data
    From the set of equations below, involving the unknowns p, q, r, and t, find the values of t / r. (Use the correct number of significant figures.)

    p = 7q
    pr = qs
    0.5pr^2 + 0.5qs^2 = 0.5qt^2


    2. Relevant equations
    ~


    3. The attempt at a solution
    I have been working on this problem for some time now. I have tried dividing all the terms by r^2 and seting (t/r)=x just to simplify the answer. I have come up with 7.5 and -7.5 but it says these answers are incorrect.
     
  2. jcsd
  3. Feb 17, 2010 #2
    Re: Simple Algebra problem that is killing me!!

    Is 3.7 one of the answers? Just making sure i did it right.
     
  4. Feb 17, 2010 #3
    Re: Simple Algebra problem that is killing me!!

    I got an answer very quickly doing simple substitutions into the bottom equation. A hint is to see if you can determine why s=7r, and then substitute this and the first equation into the bottom equation. Then use your trick that x=t/r . It will fall out nicely if you do this.
     
  5. Feb 17, 2010 #4
    Re: Simple Algebra problem that is killing me!!

    This was a lot of help. When I got to the end I got +- (7.48331). When I inputed +- (7.5) as the answer it marked it wrong. What should the answer be if this is incorrect?
     
  6. Feb 18, 2010 #5
    Re: Simple Algebra problem that is killing me!!

    Anyone?
     
  7. Feb 18, 2010 #6
    Re: Simple Algebra problem that is killing me!!

    fine t/r or t and r?
     
  8. Feb 18, 2010 #7
    Re: Simple Algebra problem that is killing me!!

    5 variables and 3 equations. one equation is second order so that probably gives 2 variables...still not enough to solve. your answer has to be in terms of a variable
     
  9. Feb 18, 2010 #8
    Re: Simple Algebra problem that is killing me!!

    I got +sqrt(56) and -sqrt(56), as you did.

    I guess the answer is needed to one significant figure, so maybe it should be +8 and -8.
     
  10. Feb 18, 2010 #9
    Re: Simple Algebra problem that is killing me!!

    I have used up two more of my answers on:
    8 and -8 it gave me this error
    "Your response is within 10% of the correct value. This may be due to roundoff error, or you could have a mistake in your calculation. Carry out all intermediate results to at least four-digit accuracy to minimize roundoff error."

    So I tried using four digits
    7.483 and -7.483 it gave me this error
    "Check the number of significant figures."

    Please help!!
     
  11. Feb 18, 2010 #10
    Re: Simple Algebra problem that is killing me!!

    5 variables, not enough equations.
     
  12. Feb 18, 2010 #11
    Re: Simple Algebra problem that is killing me!!

    The answers must be +7 and -7. Perhaps you need to use the first decimal to determine the roundoff direction. That is 7.483 ~ 7.4 ~ 7.

    If that turns out to be incorrect, then I'm stumped.
     
    Last edited: Feb 18, 2010
  13. Feb 18, 2010 #12
    Re: Simple Algebra problem that is killing me!!

    He doesn't need to sove for any particular variable, but only the ratio t/r. This turns out to be possible.
     
  14. Feb 18, 2010 #13
    Re: Simple Algebra problem that is killing me!!

    ohh thats why i asked earlier if it was t and r or t/r. I just assumed that it was t and r because everyone was giving two answers.
     
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