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Math Help - Determining slope and Y-Int

  1. #1
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    Lightbulb Determining slope and Y-Int

    So... I've been away from the math world for sometime and I need some help here.

    I have this graph:


    And I need to determine the slope and y intercept on it, can someone please tell me how I would do that in good detail so I can remember this stuff?

    Edit: Hahha I think I remembered, for slope would this be right...
    m= (6, 3) / (8, 2) = 0.5

    So would 0.5 be the slope if I used those two points?

    Still forgot how to find Yint though haha.

    Thanks!
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  2. #2
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    Keep in mind the slope is actually -0.5. Slope is rise over run (from left to right). It rises negative 1 or drops 1 and runs 2.

    m=\frac{-1}{2}

    There are a number of ways you can find the y-intercept. Let's stick with the slope intercept form.

    y=mx+b
    3=-.0.5(6)+b
    b=6

    I plugged in the coordinates (6, 3) and used the slope -0.5 that we figured out already. If those coordinates are correct, then your graph is wrong.
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  3. #3
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    So... how is my graph wrong? Lets try it out with different coordinates...

    8,2
    4,4 are the coordinates

    so we write...
    8-2=6
    4-4=0

    m = 6 ?

    then what?

    What coordinates would make this graph work?
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  4. #4
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    Quote Originally Posted by hemi View Post
    So... how is my graph wrong? Lets try it out with different coordinates...

    8,2
    4,4 are the coordinates

    so we write...
    8-2=6
    4-4=0

    m = 6 ?

    then what?

    What coordinates would make this graph work?
    You gave coordinates (6, 3)... If you meant (3, 6) [which would match your graph], then my answer is incorrect. In that case, the y-intercept would be 12. You can even see that in the graph you gave.
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  5. #5
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    Quote Originally Posted by colby2152 View Post
    You gave coordinates (6, 3)... If you meant (3, 6) [which would match your graph], then my answer is incorrect. In that case, the y-intercept would be 12. You can even see that in the graph you gave.
    Ok I see. So lets try this again here...

    M = 3, 6/ 2, 8

    = 3/6 = 2

    So slope = 2

    And Y-Int:
    y=mx+b

    6 = 2(3) + b
    b = ?


    What would b equal and are my calculations all correct in and intact with this graph I have here?

    Thanks a lot for your help btw...
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  6. #6
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    Looking at your graph, it is clear that it passes through 2 points: (0,12) and (6,0). The y-intercept is the point where x = 0, in this case you can just read it off the y-axis as 12.

    the gradient is given by the formula \frac {rise}{run} or if you find it clearer this way \frac {y_2-y_1}{x_2-x_1}

    Substituting in the points read from your graph gives
    \frac {0-12}{6-0} = -2
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  7. #7
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    thanks a lot bro!

    But I like the y = mx+b equation....

    So how would I plug in the coordinates you gave me into that and get the Y-Int of 0?

    Thank you SO much for helping!
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  8. #8
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    You know the slope is m=-2 and you have a point on the line. Use the (x, y) coordinates of a point on the line, plug the points and the slope into the equation, and chug!
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  9. #9
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    Thing is, I don't know how to "chug".. Would you mind explaining that to me in the simplest terms possible?
    Sorry for being a pest, really.


    One more question though, say I have the graph...


    and I pick the points 10, 10 and 8, 8 -- I would then write:
    m = 8 - 8/10-10 = 0 ?
    Is that possible? If not what points would make this graph work?
    Last edited by hemi; December 18th 2007 at 08:20 AM.
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  10. #10
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    thanks a lot bro!

    But I like the y = mx+b equation....

    So how would I plug in the coordinates you gave me into that and get the Y-Int of 0?

    Thank you SO much for helping!
    The y intercept is the y value where x = 0. So just substitute y = m0+b
    so y=b.



    and I pick the points 10, 10 and 8, 8 -- I would then write:
    m = 8 - 8/10-10 = 0 ?
    (x_1,y_1) = (10,10)
    (x_2,y_2) = (8,8)

    so
    m=

    so m = \frac {8-10}{8-10}
    = \frac {-2}{-2}
    =1
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