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Math Help - Helpppp!!!!

  1. #1
    Newbie
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    Dec 2007
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    Helpppp!!!!

    Heres the question exactly as is:
    Tangents to the curve f(x)=4x-x^2 intersect at (7/4,11/2). P and Q are points of tangency to the curve. Find the point of intersection of the normals drawn to the curve at points P and Q.

    I can only find the point on the left of the curve but i cant find anything about the point ont he right. helppp mee (sorta urgent unit final tomorrow).
    (p.s. the answers are (1/2,7/4) its not the anwser im after how to do it)
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  2. #2
    Eater of Worlds
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    Chaneysville, PA
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    You can find the derivative of y to find the slope, m, at the points.

    Try using y-y_{1}=m(x-x_{1})

    (4x-x^{2})-\frac{11}{2}=(4-2x)(x-\frac{7}{4})

    Solve for x. That will be the x-coordinates of the points of intersection of the two tangent lines.

    You can then find their equations and it's downhill.
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  3. #3
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    thanks i got it
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