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Math Help - points on ellipse closest to a point

  1. #1
    khh
    khh is offline
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    points on ellipse closest to a point

    Hi

    Can someone please help me with the following problem?

    Find the points of the ellipse 4x2 + 9y2 =36 that are closest to the point (1,1), as well as the point or points farthest from it?

    I have done so far:
    g(x,y)=4x2 + 9y2 - 36 = 0 ==> gx(x,y)=8x & gy(x,y)=18y

    The distance from the point is d=sqrt[(x-1)2 + (y-1)2 ]
    ==> f(x,y)= (x-1)2 + (y-1)2

    ==> fx(x,y)=
    λ.gx(x,y) and fy(x,y)=λ.gy(x,y)

    so 2(x-1) = λ . 8x and 2(y-1) = λ . 18y and from above 4x2 + 9y2 - 36 = 0

    How do I solve this?



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  2. #2
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    Re: points on ellipse closest to a point

    From 2(x-1)=8 \lambda x you can get \lambda = \frac{x-1}{4x}.

    Similarly you can get lambda in terms of y. Equate these to get an equation that will allow you to find x in terms of y or y in terms of x.

    This is where I stopped. x=\frac{9y}{5y+4} unless I messed up.

    Sub this in your ellipse equation and solve.
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  3. #3
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    Re: points on ellipse closest to a point

    Back again.

    That leads to a quartic equation which I needed my calculator to solve. Once you have your y values find the x values and don't forget to check the distance for each point you find.
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  4. #4
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    Re: points on ellipse closest to a point

    OK. So I got your PM.

    If you got as far as the quartic equation why didn't you say so?

    I don't have it written anywhere now but it's easy enough to solve, using the Newton Raphson method.

    Post it if you like and I'll check it.

    Do you know the Newton Raphson method?
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