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Math Help - Just a little question i need help with.

  1. #1
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    Just a little question i need help with.

    1. The curve C is given by the equation y=mx3-x2+8x+2, for a constant m
    A) find dy/dx

    The point P lies on C, and has the x-value 5
    The normal to C at P is parallel to the line given by the equation y+4x-3=0.

    B)find the gradient of the curve at C at P.

    Hence find:
    C) (I) the value of m,

    (ii) the y-value at p.

    For A) i got:
    dy/dx=3mx2-x+8


    I'm not really sure how to carry on with it now any help would be brilliant
    Thanks.
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  2. #2
    MHF Contributor MarkFL's Avatar
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    St. Augustine, FL.
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    Re: Just a little question i need help with.

    a) Check your differentiation. Your second term is wrong.

    b) Evaluate \frac{dy}{dx}\left|_{x=5}

    c) The statement "The normal to C at P is parallel to the line given by the equation y+4x-3=0" tells us that the derivative of y with respect to x at x = 5 is the negative reciprocal of the slope of the given line. So, find the slope of the line y+4x-3=0, invert it, negate it and equate this to \frac{dy}{dx}\left|_{x=5}.

    i) Solve the resulting equation for m.

    ii) We are told the xcoordinate of point P is 5, so to find the y-coordinate evaluate y(5).
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  3. #3
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    Re: Just a little question i need help with.

    Thanks i was really stuck on that question
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