I'm not allowed to use a calculator on this one....

I have to get a cubic equation from a set of points but they don't have a constant D3.

I completely forget how to do this...

the set of data are: left is x right is y

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- Feb 22nd 2009, 12:17 PMroloddtacquiring a cubic equation from a data table
I'm not allowed to use a calculator on this one....

I have to get a cubic equation from a set of points but they don't have a constant D3.

I completely forget how to do this...

the set of data are: left is x right is y - Feb 22nd 2009, 12:37 PMHallsofIvy
Any cubic equation is of the form $\displaystyle y= Ax^3+ bx^2+ cx+ d$. One way to get a cubic equation from the data is to just pick three points and put their x and y values into that, so you have 3 equations to solve for a, b, and c. If I were doing this I would pick numbers at the beginning, center and end of the list.

Or you might be expected to give the "least squares" cubic approximation. That's much harder. - Feb 22nd 2009, 12:42 PMroloddt