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Math Help - I forgot how to do this...help.

  1. #1
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    I forgot how to do this...help.

    Prove that there is no line through (-1,2) that is tangent to the graph of f (x) = x^2+ 3x .
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  2. #2
    Senior Member
    Joined
    Jul 2009
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    Hi Arturo_026

    The line through (-1, 2) : y = mx + c

    Subs. (-1, 2) to the equation gives c = 2 + m, so the equation of the line : y = mx + 2 + m

    If the line is tanget to the curve, then they will have common point, so :
    mx + 2 + m = x^2 + 3x, where m = f '(x) = 2x + 3

    Then...
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  3. #3
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    May 2009
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    Hahaha.
    It is pretty simple. The problem is that I was kinda nervous for the final that I was getting my memories black outs
    But i just tookk it and I see it now.
    Thanks for replying.
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