1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Simple Algebra problem that is killing me
Loading...