# How to fit custom equation in a linear regression?

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Rik
on 18 Nov 2022

### Answers (1)

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.

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