Microsoft excel中的LINEST函数； 曲线拟合三二次方程

Question

Im trying to curve fit the first graph in the set of four using LINEST. This curve needs to be a graph of 3, possible 4, variables: Suction Pressure (SP), OD Temp, ID WB Temp, and possible ID DB Temp (other graphs didnt need the DB to work, may not be needed here). Isnt a simple LINEST, but a tri- or quad-quadratic LINEST.

Attempted: 2nd, 3rd degree LINEST fits using corresponding x,y values from first graph.

How do i do this?

Answer 1

Ok if you have the data in table format you can use the following formula

=LINEST(knownys, knownxs^{1, 2, 3})

When you enter it select 4 cells across, put in the formula and enter with ctrl+shift+enter . The values it returns would be A1,A2,A3, and b going left to right if the formula has the following equation: y=A1*x^3+A2*x^2+A3*x+B.

Another option is to fit curves using the chart trendline and you can get the exponents from there.

Source

Edit

Based on your edits I would suggest you find a better statistical software then excel but one option would be to use solver. You would set up a bunch of cells with the constants you wanted a1,a2,b1,b2 etc.

Then you would have your columns with your independent variables (OD temp, ID WB Temp, etc) a column with your known ys, a column with your calculated ys and an error column (known y - calcy)^2 you would sum up the error column and run solver to minimize the sum of the error.

calc y would be something like =a1*ODtemp^2+a2*ODtemp+b1*WBtemp^2+b2*WBtemp+C or whatever form your formula needs to fit.