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Math Help - Parabola rotation

  1. #1
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    Parabola rotation

    A parabola with equation y = ax^2 is graphed in the standard coordinate plane. Suppose that the parabola is rotated clockwise by 90 degrees about the origin. Which of the following is an equation for the parabola that results from this rotation?

    A. x = -ay^2
    B. x = ay^2
    C. y = -ax^2

    The answer should be B but why can't it be A?

    I thought that 90 deg. clockwise rotation about the origin was of form: (x,y)-->(y, -x) but by substitution I obtain choice A not B...
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  2. #2
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    Re: Parabola rotation

    (x,y)-->(y,-x) gets you an anti-clockwise rotation. You need to use (x,y)-->(-y,x).
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  3. #3
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    Re: Parabola rotation

    Hi,
    You are correct in your description of a clockwise 90 degree rotation about the origin. The attachment gives an argument for why choice b is the correct answer. I hope it is understandable and that it helps for this and similar problems.

    Parabola rotation-mhfalgebra3.png
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