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Is it possible to use these points: (4,-2),(9,-8)

and write an equation in slope intercept form, without first writing an equation in point-slope form, and then rewriting it to slope-intercept form?

  • Math -

    sure, first find the slope

    slope = (-8+2)/(9-4) = -6/5

    then the equation is y = (-6/5)x + b
    sub in (4,-2) , or the other point
    -2 = (-6/5)(4) + b
    I would now multiply by 5 (since I try to avoid fractions as much as possible)
    -10 = -24 + 5b
    b = 14/5

    so y = (-6/5)x + 14/5

  • Math -

    No, you have to first use
    y - y1 = m(x - x1) [point slope form]
    and then rewrite in the form
    y = mx + c [slope intercept form]

    When you are given 2 points you have to find the slope first
    [m = (y2 - y1)/(x2 - x1)]
    then sub in the slope with either of the points in the equation in point slope form.

    Hope that helps.

  • Math -

    I'll correct myself, yes you can I didn't think of the above method by Reiny.
    I never used that way before :)
    It's quite useful though.

    Sorry for my error

  • Math -

    why do you have to sub in (4,-2)? why can't you use 9,-8)?

  • Math -

    As I said, you can use either point.
    I usually use the point that has the "nicer" numbers. Either point will give you the same value of b

    A good idea is to then use the other point to see if the equation works.

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