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: Algebra help for physics

  1. Oct 3, 2006 #1
    Hey guys, I'm having an algebra blank, can't figure this out for some reason, I know it simple. aHH!. Ok.. heres the question.

    I'm trying to isolate d truck, ie, trying to solve for dtruck from the following.

    Dtruck = Dtruck + 100m
    75km/hr 88km/hr

    I think you multiply 75km/hr to the right side, but there are 2 Dtrucks, and i forget how to manipulate that. Once again, I want only Dtruck on the left side.. so Dtruck = xxxxxx. Thanks for the help.
     
  2. jcsd
  3. Oct 3, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I assume this is meant to be two fractions equal to each other? If so, just cross multipy:
    [tex]\frac{A}{B} = \frac{C}{D}[/tex]
    becomes:
    [tex]A \times D = B \times C[/tex]

    Once you do that, multiply everything out and isolate Dtruck.
     
  4. Oct 3, 2006 #3
    thanks.. but still not totally getting it don't think. If i do that.. I get this right?

    (dtruck)(88km/hr) = (dtruck + 110m)(75km/hr)

    (dtruck)(88km/hr) = (dtruck)(75km/hr) + (110m)(75km/hr)

    Now i'm stuck again.. i feel stupid. I know I should be able to islolate dtruck, but how.
     
  5. Oct 3, 2006 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Subtract (dtruck)(75km/hr) from each side.
     
  6. Oct 3, 2006 #5
    but wouldn't that take away the dtruck that i want?

    (dtruck)(88km/hr) - (dtruck)(75km/hr)

    wouldn't that take away the dtruck?.. and just leave 10km/hr. That can't be right.
     
  7. Oct 3, 2006 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Nope. Consider this: aX - bX = (a - b)X
     
  8. Oct 3, 2006 #7
    Doc, you are a genius. One of a kind. Thank you.. i really appreciate it. I should have already known that.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook