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Thread: Equation of a plane

  1. #1
    Forum Admin topsquark's Avatar
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    Equation of a plane

    This is a post from CarissaF.

    Quote Originally Posted by CarissaF View Post
    if f(x,y)=4x^2+5xy+2y^2
    Then an implicit equation for the tangent plane to the graph at point (2,-3) is:


    I got an answer of x-2y=0 but it is wrong... I can't figure out why. This is my work.


    0=f(2,-3)+fx(2,-3)(x-2)+fy(2,-3)(y--3)
    f(2,-3)=4
    fx(2,-3)=1
    fy(2,-3)=-2


    0=4+1(x-2)-2(y+3)
    0=4+x-2-2y-6
    4=x-2y


    Can anyone spot my mistake?
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  2. #2
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    Re: Equation of a plane

    Quote Originally Posted by topsquark View Post
    This is a post from CarissaF.
    Consider this $0=f_x(2,-3)(x-2)+f_y(2,-3)(y+3)$
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