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### How to determine whether a table of x-y values forms a quadratic relationship?

In order to determine whether a table of x-y values forms a quadratic relationship between the x and y variables or not, get the second differences between the given set of y-values. If the second differences between each consecutive pair of y-values is equal, then the given table of x-y values forms a quadratic relationship between the x and y variables.

For example, in the following table of x-y values, the second differences between each pair of y-values are constant for all y-values:
XYFirst difference, Fn = Yn - Yn-1Second difference, Sn = Fn - Fn-1
211
42727 - 11 = 16
65151 - 27 = 2424 - 16 = 8
88383 - 51 = 3232 - 24 = 8

The second difference of each pair of corresponding y-values is 8. Since the second difference is same for all corresponding y-values of the table, therefore it represents a quadratic relationship between x and y. The quadratic equation for the above table is $y={x}^{2}+2\,x+3$

On the other hand, if a table of x-y values does not represent a quadratic relationship, the values in the second difference column will differ.