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For what value of b will the line y=-2x+b be tangent to the parabola y=3x^2+4x-1 ? (Show your work - 2 marks)

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Alex
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  1. I love it when you guys as us to show our work. Ever think of showing some of yours?

    y' = 6x+4
    The line given has slope -2.
    So, when do we have

    6x+4 = -2?
    x = -1

    y(-1) = -2, so we want the point (1,-2) to be on the line. That means

    -2 = -2(-1)+b
    b = -4

    See the graphs at

    http://www.wolframalpha.com/input/?i=plot+y%3D3x^2%2B4x-1%2C+y%3D-2x-4

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    Steve

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