Finding equation of a curve through 5 points

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Discussion Overview

The discussion revolves around finding an equation of a curve that passes through five specific points in a Cartesian plane. Participants explore various approaches to determine the curve, including polynomial equations and interpolation methods, while considering the nature of the function that could fit the points.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests using a general polynomial equation of the form ax^4 + bx^3 + cx^2 + dx + e, but expresses concern about the complexity of solving for coefficients with five points.
  • Another participant questions whether the function must be a polynomial, highlighting that other types of functions (exponential, logarithmic, etc.) could also fit the points.
  • It is noted that there are infinitely many functions that can pass through the five points, but there is exactly one fourth-order polynomial that can do so.
  • A participant introduces the "Legendre polynomial formula" as a method to derive the polynomial that fits the points, providing a detailed expression for it.
  • Another participant mentions that the problem may be approached as a linear regression issue, suggesting the possibility of outliers in the data affecting the results.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the function that should be used to fit the points, with some advocating for polynomial solutions while others suggest considering alternative functions. The discussion remains unresolved regarding the best approach to take.

Contextual Notes

There are limitations regarding assumptions about the type of function that can be used, and the complexity of solving for coefficients in polynomial equations is acknowledged. The discussion also hints at potential issues with the data itself.

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I'm trying to work out the equation of a curve through 5 very specific points... these points are
(47.5, 46.3) (45,48.1) (43, 38.1) (40, 32.8) (37, 26.5)

Thought of having general equation ax^4+bx^3+cx^2+dx+e and subbing all the points in and solving but that would take way too long... If there is any simper way to do this or if you could post any link to a program capable of solving it quickly that'd be great.
 
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Hi,

Does the problem specify if the function is a polynomial, exponential, logarithmic, trigonometric, hyperbolic, square root?

You can't simply assume the function is a polynomial.
 
The point is that there exist an infinite number of functions, whether polynomials, logarithms, etc. that will pass through any given 5 points. There exist exactly one fourth order polynomial through 5 points, if that's what you want. If you don't like solving 5 linear equations for the 5 polynomials, try using the "Legendre polynomial formula": The polynomial passing through the five points (x_0, y_0), (x_1, y_1), (x_2, y_2), (x_3, y_3), (x_4, y_4) is given by
\frac{(x- x_1)(x- x_2)(x- x_3)(x- x_4)}{x_0- x_1)(x_0- x_2)(x_0- x_3)(x_0- x_4)}+ \frac{(x- x_0)(x- x_2)(x- x_3)(x- x_4)}{x_1- x_0)(x_1- x_2)(x_1- x_3)(x_1- x_4)}+ \frac{(x- x_0)(x- x_1)(x- x_3)(x- x_4)}{x_2- x_0)(x_2- x_1)(x_2- x_3)(x_2- x_4)}+ \frac{(x- x_0)(x- x_1)(x- x_2)(x- x_4)}{x_3- x_0)(x_3- x_1)(x_3- x_2)(x_0- x_4)}+ \frac{(x- x_0)(x- x_1)(x- x_2)(x- x_3)}{x_4- x_0)(x_4- x_1)(x_4- x_2)(x_4- x_3)}

That is, each fraction has, in the numerator, x minus all x-values except one and, in the denominator, that x-value minus each of the others.
 
Just looking at the graph makes me think at a simple problem of linear regression with an outlier. Are you sure that there is no bug in the data ?
 

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