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Changing format of the equation of interpolation

  1. Oct 1, 2015 #1
    Give 2 points: (a, f(a)) and (b, f(b)), is possible plot a line function L(x) that intersects these two points, the linear function is:

    [tex]L(x) = f(b) \frac{(x - a)}{(b-a)} + f(a) \frac{(x-b)}{(a-b)}[/tex]

    In other format, is:

    [tex]L(x)-f(a) = \frac{f(b)-f(a)}{(b-a)} (x-a) [/tex]

    Now, given 3 points: (a, f(a)), (m, f(m)) and (b, f(b)), is possible too plot a quadratic function P(x) that intersects these three points, and the quadratic function is:


    And what I REALLY WANT is change the format of the function P(x) for a new format of equation where all factors are the form (u-v).

    Here are some relationships that can help you to help me:

    Interpolação de segunda ordem (2D).png nth derivative.png

    Equation of hyperbola:
    [tex] \left( \frac{x}{a} \right)^2 - \left( \frac{y}{b} \right)^2 = 1 [/tex]
    Equation of elipse:
    [tex] \left( \frac{x}{a} \right)^2 + \left( \frac{y}{b} \right)^2 = 1 [/tex]
    Equation of parabola:
    [tex] \left( \frac{x}{a} \right)^2 + \left( \frac{y}{b} \right) = 1 [/tex]
    Equation of straight line:
    [tex] \left( \frac{x}{a} \right) - \left( \frac{y}{b} \right) = 1 [/tex]

    This is a horrible question, because involves so much algebraic operations, for this reason I posted all these relationship above, because, maybe you can to get the answer for my question without make counts, just comparing the symmetry...
  2. jcsd
  3. Oct 1, 2015 #2


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    Science Advisor

    It is not clear in what way what you want is different from the display.
  4. Oct 1, 2015 #3
    Realize that the 2nd equation is of the form: (u-v) = (u-v)/(u-v) * (u-v), all factors are of the form (u-v). This is not true for the 1nd equation, that is of the form w = w (u-v)/(u-v) + w (u-v)/(u-v), the same happens with the 3nd equation, I would like of rewrite all factors in the form (u-v), obviously, P(x), f(a), f(m) and f(b) are separated factors.
  5. Oct 2, 2015 #4
    EDIT: I got my answer!

    Calling a of x3, m of x2 and b of x1 and f(a) of y3, f(m) of y2 and f(b) of y3, the answer for my question is:

    [tex](y - y_1) = \frac{(y_3 - y_1)}{(x_3 - x_2)(x_3 - x_1)} (x-x_2)(x-x_1) + \frac{(y_2 - y_1)}{(x_2 - x_1)(x_2 - x_3)} (x-x_1)(x-x_3)[/tex]

    BUT, my second question is: is possible isolate all the y terms in the left side of equation and isolate all the x terms in the right side? (independent of the format of equation, the important, for my is just isolate the terms x and y) If yes, so, you can help me make this?
    Last edited: Oct 2, 2015
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