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Homework Help: Linear approximation

  1. Oct 10, 2012 #1
    1. The problem statement, all variables and given/known data
    Hey guys I'm having a hard time understanding how the book obtained the solution.

    Here is the question

    A function f is given along with a local linear approximation of L to f at a point P. Use the information given to determine point P.

    f(x,y)= x2+y2; L(x,y)=2y-2x-2

    Formula for local linear approximation is


    So plugging in my known values


    I was able to get up to here, and then when I looked at the solution in the book, it simply said x0=-1 y0 =1 and it isn't obvious to me why.

    Any help is appreciated

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 10, 2012 #2


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    Science Advisor
    Homework Helper
    Gold Member

    That isn't correct.

    That is the correct linear approximation near ##(x_0,y_0)##. Buy why do you set it equal to your original ##f(x,y)##? The linear approximation is an approximation to ##f(x,y)## but not equal to it unless ##f(x,y)## is linear itself. But you are given the formula for the linear approximation, which I have highlighted in red. Set it equal to that and see if you can see how they got that answer.
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