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 1 question

  1. Jun 28, 2005 #1
    If 3x+4y=7 and 2y=6x+6, then what is xy?


    3X+4y=7
    -6X+2y=6

    (2)3X+4y=7 which goes to 6X+8Y=14

    I cross the X out, subtract and get 6y=8

    then I got 4/3
    and final answer i got was 20/3

    HOWEVER, the answer says is is -2/3 ... I don't understand how that is the answer...

    Please Help!
     
  2. jcsd
  3. Jun 28, 2005 #2

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    You should add them.y=20/10=2.

    Daniel.
     
  4. Jun 28, 2005 #3
    i don't understand...
     
  5. Jun 28, 2005 #4

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    [tex] \left\{\begin{array}{c} 6x+8y=14 \\ -6x+2y=6 \end{array} \right [/tex]

    As i said before,add the two equations,so you'd eliminate "x".

    Daniel.
     
  6. Jun 28, 2005 #5
    yeah, then i got 6y=8
     
  7. Jun 28, 2005 #6

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    Why?Don't ypu know what "addition" or "adding" means...?

    Daniel.
     
  8. Jun 28, 2005 #7
    oh, my bad ....2, so do I plug it back in, sorry I am going into 7th grade, learning this from a book.

    update: ok I understand what you said, now I have y=2...
     
    Last edited: Jun 28, 2005
  9. Jun 28, 2005 #8
    Yes, you are correct that y=2. Now as you said plug it back in to find the unknown x.

    [tex]3x + 4y = 7[/tex]

    You know what to do.

    Jameson
     
  10. Jun 28, 2005 #9
    thank you, dextercioby and Jameson, I understand the problem now.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook