Math Help!!

Find all points of the graph of y=7x^2+7x whose tangent lines are perpendicular to the line -49y=x

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asked by Andrew
  1. y = -1/49 x

    the slope of the perpendicular is
    ... -(-1/49) = 49

    the 1st derivative is the slope of the tangent ... dy/dx = 14x + 7

    14x + 7 = 49

    find x, then find y from the original function

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    posted by Scott
  2. -49y=x has slope -1/49
    so, you want lines with slope 49

    since y' = 7(2x+1), that means x=3.
    y(3) = 84

    so, the line is

    y-84 = 49(x-3)

    See

    http://www.wolframalpha.com/input/?i=plot+y%3D7x%5E2%2B7x,+y%3D49(x-3)%2B84+for+x%3D0..5

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    posted by Steve

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