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How to find the coordinates of a stationary point? PLEASE HELP?

The curve y=4x^2 lnx has one stationary point. Find the coordinates of this stationary point, giving your answers correct to 3 decimal places. Then determine whether this point is a maximum or a minimum point.

Please help, for 10 points?

1 Answer

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  • Henry
    Lv 5
    9 years ago
    Favorite Answer

    Stationary points are where dy/dx = 0, makes sense in thinking about a maximum or minimum, in order for them to be a maximum or minimum, then their gradient must be zero.

    In this example, you're going to have to differentiate using the product rule, as it's in the form:

    y = uv

    dy/dx = vu' + uv'

    so y = 4x^2lnx where u = 4x^2 and v = ln x note that the differential of lnx is 1/x

    and dy/dx = 8xlnx + (4x^2 x 1/x)

    = 8xlnx + 4x

    dy/dx = 0 so.... 8xlnx = -4x

    2xlnx = -x

    2lnx = -1

    lnx = -1/2

    x = e^-1/2 = 0.607

    and so y = 4(0.607)^2 x ln0.607 = -0.736

    To determine whether it's a maximum or minimum stationary point, consider the following:

    If the second differential, that is, the changing gradient, is positive, then the line is on its way up, it was at a minimum point, whereas if the changing gradient is negative, then the line is on its way down, it was at a maximum point.

    so we had dy/dx = 8xlnx + 4x

    d^2y/dx^2 = 8lnx + 8 + 4

    substitue in the value of x of 0.607

    d^2y/dx^2 = 8ln(0.607) + 8 + 4 = 8

    This is positive, therefore the stationary point is a minimum point.

    Hope this was of help :)

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