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Math Help - Solving equations graphically.

  1. #1
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    Solving equations graphically.

    Before I start, if this is the wrong section, I'm sorry, I don't really know which section to post it on.

    How do I solve these equations graphically :

    x+2y=7
    x+y=3

    This is how I work it out, but for some reason I'm not getting the correct answer, even though I've used it to solve other equations like this :

    x+2y=7
    Let x=0
    0+2y=7
    2y=7
    y=3.5
    Co-ordinate points=(0,3.5)

    x+2y=7
    Let y=0
    x+2(0)=7
    x=7
    Co-ordinate points=(7,0)

    x+y=3
    Let x=0
    0+y=3
    y=3
    Co-ordinate points=(0,3)

    x+y=3
    Let y=0
    x+0=3
    x=3
    Co-ordinate points=(3,0)


    So, as we can see, the points for the first equation the co-ordinate points I plot are (0,3.5) and (7,0) and for the second equation I plot (0,3) and (3,0).

    The problem I'm having is when I plot them on the graph, they don't intersect, which means I don't have an answer. If they don't intersect, it must be wrong, but how, I think I've done everything correctly? Also, I've looked at other examples in my maths textbook and they work, but for some reason this one doesn't.

    Please, please help. I have a test tomorrow and I really need to know how to do this.
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  2. #2
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    Re: Solving equations graphically.

    They do intersect, just not in the first quadrant.
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  3. #3
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    Re: Solving equations graphically.

    How do I solve these equations graphically :

    x+2y=7
    x+y=3
    x+2y = 7

    change to slope-intercept form ...

    2y = -x + 7

    y = -\frac{x}{2} + \frac{7}{2}


    x+y = 3

    change to slope-intercept form ...

    y = -x+3


    graph them ...
    Attached Thumbnails Attached Thumbnails Solving equations graphically.-linearsystem.png  
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