# How to fit custom equation in a linear regression?

7 views (last 30 days)
John Carlo on 18 Nov 2022
Commented: Rik on 18 Nov 2022
I'd like to fit a custom equation for linear regression. For example y = a1 + a2 * x^4 + a3 * x^5. How do you do this with fitlm function?
Rik on 18 Nov 2022
And the other questions I asked in my comment? Did you try the code generation tool? What fit options did you use?

cr on 18 Nov 2022
Edited: cr on 18 Nov 2022
Presuming you are trying to estimate a1, a2, a3 you can just to a lmdivide. You will need data for x & y with atleast 3 to solve (and 4 for a linear regression).
e.g. with the n observations, you will have n equations a1 + a2xi^4 + a3xi^5 = yi
do
A = X\Y
where A is the 3 x 1 vector of unknowns a1 a2 a3
X is n x 3 matrix of observations x1, x2, x3,...xn
[1 x1^4 x1^5;
1 x2^4 x2^5; ....;
1 xn^4 xn^5]
Y is n x 1 vector
[y1; y2;..;yn]
Regards.
##### 2 CommentsShowHide 1 older comment
cr on 18 Nov 2022
Edited: cr on 18 Nov 2022
You may use fitlm(X,Y). Note that \ or fitlem can be used only for linear regression and not any equation in general. You will need other functions like nlinfit etc for any equation in general.
Regards.

### Categories

Find more on Get Started with Curve Fitting Toolbox in Help Center and File Exchange

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!