How to calculate r squared from a given cos regression model

Hi i am a new learner, i got confused how can i calculate like this,

if there's two sets,

x = [0.0000 0.5236 1.0472 1.5708 2.0944 2.6180 3.1416]

y = [2.7600 -2.3902 1.3800 0.0000 -1.3800 2.3902 -2.7600]

and the given function, y = a ∗ cos(5x), find the r squared for this function

Answer 1

[Rik]

y_fit = a ∗ cos(5*x) 
err=y-y_fit;
SSres=sum(err.^2);
SStot=sum((y-mean(y)).^2);
rsq=1-(SSres./SStot);

[Rey Li]

Hi, thanks for your answer, wondering how to know "a" ?

[Rik]

You either already know the value, or your question is how to fit this function to your data.

What did you try so far? Do you have the Curve Fitting Toolbox available? Is this a homework assignment?

[Rey Li]

Sorry for the confusion, the question is to fit this function and find the R2 value for this function when the sum of squared residuals is minimised. Yes i have fitting toolbox i will try to figure it out, thanks for your help

[Rik]

Hint: start your looking with the fit function.

[Rey Li]

thanks :) i've known how to do it now