Page 1 of 2 12 LastLast
Results 1 to 15 of 22

Math Help - System of equations

  1. #1
    Junior Member
    Joined
    Jan 2008
    Posts
    27

    System of equations

    I am working on system of equations, and I am totally lost, every time I do it it turns out really wrong. Could some one help explain this to me.

    Find the vaules of x and y that solve the following systems of equations:

    -7x+9y=-2
    -3x+2y=1

    Thanks!
    Follow Math Help Forum on Facebook and Google+

  2. #2
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    Quote Originally Posted by GeinoD View Post
    I am working on system of equations, and I am totally lost, every time I do it it turns out really wrong. Could some one help explain this to me.

    Find the vaules of x and y that solve the following systems of equations:

    -7x+9y=-2
    -3x+2y=1

    Thanks!
    there are two main ways to do problems like these. there is elimination and substitution. i will describe elimination, since it is generally what i prefer to do

    first we want to get the coefficient of one of the variable the same in magnitude in both equations. then we can add or subtract one equation from another (depending on signs) to eliminate that variable. once we get rid of that variable, only one of the variables are left which we can easily solve for. we solve for it, and then use it to find the other variable that we eliminated originally.

    let's say we want to eliminate the x's

    first step, multiply equation (1) by 3, and equation (2) by -7

    what to you get?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Jan 2008
    Posts
    27
    I'm Sorry I think we are supposed to use Substitution to solve the system, if that sounds about right.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    Quote Originally Posted by GeinoD View Post
    I'm Sorry I think we are supposed to use Substitution to solve the system, if that sounds about right.
    ...ok. in that case. solve for y from equation (2) and plug it into equation (1) so you can solve for x. then back-substitute to solve for y
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Junior Member
    Joined
    Jan 2008
    Posts
    27
    This is what I did.

    -3x+2y=1
    +3x +3x
    2y=3x+1
    /2 /2 /2
    y=3/2x+1/2

    -7x+9(3/2x+1/2)=-2

    this is where I get into trouble...

    -7x+27/2x+9/2=-2

    At this point I think I am way off.
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Junior Member
    Joined
    Jan 2008
    Posts
    27
    -7x+27/2x+9/2=-2

    -7x+27/2x+18

    13/2x=-20
    x=53/2
    Follow Math Help Forum on Facebook and Google+

  7. #7
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    Quote Originally Posted by GeinoD View Post
    This is what I did.

    -3x+2y=1
    +3x +3x
    2y=3x+1
    /2 /2 /2
    y=3/2x+1/2

    -7x+9(3/2x+1/2)=-2

    this is where I get into trouble...

    -7x+27/2x+9/2=-2

    At this point I think I am way off.
    no, you're right so far. continue. solve for x now
    Follow Math Help Forum on Facebook and Google+

  8. #8
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    Quote Originally Posted by GeinoD View Post
    -7x+27/2x+9/2=-2

    -7x+27/2x+18

    13/2x=-20
    x=53/2
    you lost me at the line i made red. your answer is incorrect...what happened?
    Follow Math Help Forum on Facebook and Google+

  9. #9
    Junior Member
    Joined
    Jan 2008
    Posts
    27
    -3x+2y=1
    +3x +3x
    2y=3x+1
    /2 /2 /2
    y=3/2x+1/2

    -7x+9(3/2x+1/2)=-2



    -7x+27/2x+9/2=-2

    -7/1x+27/2+9/2=-2

    -14/2x+27/2x+9/2=-2

    -13/2x+9/2=-2

    9/2 9/2

    -13/2x=9
    -13/2 -13/2

    x=18/13
    Follow Math Help Forum on Facebook and Google+

  10. #10
    Junior Member
    Joined
    Jan 2008
    Posts
    27
    When I put 18/13 in place of x in the next equation I end up with y=312/13
    Follow Math Help Forum on Facebook and Google+

  11. #11
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    Quote Originally Posted by GeinoD View Post
    -3x+2y=1
    +3x +3x
    2y=3x+1
    /2 /2 /2
    y=3/2x+1/2

    -7x+9(3/2x+1/2)=-2



    -7x+27/2x+9/2=-2

    -7/1x+27/2+9/2=-2

    -14/2x+27/2x+9/2=-2

    -13/2x+9/2=-2

    9/2 9/2

    -13/2x=9
    -13/2 -13/2

    x=18/13
    Quote Originally Posted by GeinoD View Post
    When I put 18/13 in place of x in the next equation I end up with y=312/13
    you need to review how to solve equations. your solution is again wrong. the correct answer is: x = -1. now use that to find y

    (you can check your answer you know. when you got x = 18/13, why didn't you plug it into the left hand side of the (red) equation and see if you actually got -2 as the answer?)
    Follow Math Help Forum on Facebook and Google+

  12. #12
    Junior Member
    Joined
    Jan 2008
    Posts
    27
    So is the red one where I went wrong?
    Follow Math Help Forum on Facebook and Google+

  13. #13
    Junior Member
    Joined
    Jan 2008
    Posts
    27
    anyone? I'm totally freaking out because at this rate it will take me 3 days to finish. What am I doing wrong, I just dont get it and there is no one I can ask for help from
    Follow Math Help Forum on Facebook and Google+

  14. #14
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    Quote Originally Posted by GeinoD View Post
    So is the red one where I went wrong?
    no. that's not where you went wrong. it is the line you should have checked your answer in

    you went wrong 3 lines after that. you should have subtracted 9/2 from both sides, but you did something else, i'm not even sure what
    Follow Math Help Forum on Facebook and Google+

  15. #15
    Junior Member
    Joined
    Jan 2008
    Posts
    27
    Ok, how about this

    -7x+9y=-2
    -3x+2y=1

    3(-7x+9y)=-2*3
    -21x+27y=-6

    7(-3x+2y)=1*7
    -21x+14y=7

    -42x+41y=1
    Follow Math Help Forum on Facebook and Google+

Page 1 of 2 12 LastLast

Similar Math Help Forum Discussions

  1. system of equations when x,y and z = 1
    Posted in the Advanced Algebra Forum
    Replies: 4
    Last Post: May 3rd 2011, 09:33 AM
  2. Replies: 2
    Last Post: April 20th 2010, 04:26 PM
  3. Replies: 4
    Last Post: January 18th 2010, 05:08 PM
  4. System of Equations - Help
    Posted in the Algebra Forum
    Replies: 1
    Last Post: November 3rd 2008, 10:30 AM
  5. system of equations
    Posted in the Algebra Forum
    Replies: 2
    Last Post: April 12th 2007, 07:13 AM

Search Tags


/mathhelpforum @mathhelpforum