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Math Help - System of Equation

  1. #1
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    System of Equation

    Solve this system by graphing: y= x - 1
    y= 9 - x

    I understand the first one; y= x - 1. My first point would be at -1, then I rise 1 point and run 1 point (x always has a 1 in front of it), and that's my second point.

    I get that.

    I don't get the second one though; y= 9 - x

    Is x even suppose to be there? Do I move it? Switch it around with 9?

    Here's what I think. Leaving the equation just as it is; my first point on the graph would be at -1, then I would rise 9 and run 1, that'll be my second point.

    BUT!

    When I graph that my solution is (0,1), but it's suppose to be (5,4).

    So I've gone wrong some where...any ideas how to do this!

    Much appreciated.
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by amai View Post
    Solve this system by graphing: y= x - 1
    y= 9 - x

    I understand the first one; y= x - 1. My first point would be at -1, then I rise 1 point and run 1 point (x always has a 1 in front of it), and that's my second point.

    I get that.

    I don't get the second one though; y= 9 - x

    Is x even suppose to be there? Do I move it? Switch it around with 9?

    Here's what I think. Leaving the equation just as it is; my first point on the graph would be at -1, then I would rise 9 and run 1, that'll be my second point.

    BUT!

    When I graph that my solution is (0,1), but it's suppose to be (5,4).

    So I've gone wrong some where...any ideas how to do this!

    Much appreciated.
    Remember that old Junior High Math stuff where you were told about the Commutative Law of Addition? That means that
    y = 9 - x = -x + 9

    This is in slope-intercept form. So start with the y-intercept: (0, 9). The slope is -1, so we know there is another point on this line at (1, 8). Draw your line between these two points.

    -Dan
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  3. #3
    littledog1
    Guest

    system of equation.

    Quote Originally Posted by amai View Post
    Solve this system by graphing: y= x - 1
    y= 9 - x

    I understand the first one; y= x - 1. My first point would be at -1, then I rise 1 point and run 1 point (x always has a 1 in front of it), and that's my second point.

    I get that.

    I don't get the second one though; y= 9 - x

    Is x even suppose to be there? Do I move it? Switch it around with 9?

    Here's what I think. Leaving the equation just as it is; my first point on the graph would be at -1, then I would rise 9 and run 1, that'll be my second point.

    BUT!

    When I graph that my solution is (0,1), but it's suppose to be (5,4).

    So I've gone wrong some where...any ideas how to do this!

    Much appreciated.



    Not sure I'm understanding your problem but it sounds like your trying to figure out slope. First start by picking a value for "X" eg. 2 and enter it in the first linear equation y=x-1 it would become y=(2)-1; y=1... so your first point on this graph is 2,1 then you would select your second value for "X" eg. 8 and then enter it in the equation again y=x-1 it would become y=(8)-1; y=7 so your second point on the graph is 8,7 so now that you have two points on your graph take y1-y2 divided by x1-x2 . It would look like this. 1-7/2-8 which equals -6/-6 so your slope is one.

    your second equation is the same thing pick to values for x mark down your coordinates and then do y1-y2 divided by x1-x2 and you will have your slope. Here is a link as well.
    Slope of a Straight Line
    Last edited by littledog1; February 12th 2007 at 07:06 PM. Reason: wrote equation backwards sorry
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