Use a trendline formula to get values for any given X with Excel Is there an easy way to apply the trend line formula from a chart to any given X value in Excel?
For example, I want to get the Y value for a given X = $2,006.00. I've already taken the formula and retyped it out be:
=-0.000000000008*X^3 - 0.00000001*X^2 + 0.0003*X - 0.0029

I am continually making adjustments to the trend line by adding more data, and don't want to retype out the formula every time.

 A: Try trend(known_y's, known_x's, new_x's, const).
Column A below is X.  Column B is X^2 (the cell to the left squared).   Column C is X^3 (two cells to the left cubed).  The trend() formula is in Cell E24 where the cell references are shown in red.
The "known_y's" are in E3:E22
The "known_x's" are in A3:C22
The "new_x's" are in A24:C24
The "const" is left blank.
Cell A24 contains the new X, and is the cell to change to update the formula in E24
Cell B24 contains the X^2 formula (A24*A24) for the new X
Cell C24 contains the X^3 formula (A24*A24*A24) for the new X
If you change the values in E3:E22, the trend() function will update Cell E24 for your new input at Cell A24. 

Edit ====================================
Kirk,
There's not much to the spreadsheet.  I posted a "formula view" below.
The "known_x" values are in green in A3:C22
The "known_y" values are in green in E3:E22
The "new_x" values are in cells A24:C24, where B24 and C24 are the formulas as shown.  And, cell E24 has the trend() formula.   You enter your new "X" in cell A24.
That's all there is to it.

A: Use LINEST, as shown:

The method is to create new columns (C:E here) containing the variables in the fit.  Perversely, LINEST returns the coefficients in the reverse order, as you can see from the calculation in the "Fit" column.  An example prediction is shown outlined in blue: all the formulas are exactly the same as for the data.
Note that LINEST is an array formula: the answer will occupy p+1 cells in a row, where p is the number of variables (the last one is for the constant term).  Such formulas are created by selecting all the output cells, pasting (or typing) the formula in the formula textbox, and pressing Ctrl-Shift-Enter (instead of the usual Enter).
