Question

I am trying to plot a function and its derivative using fplot.

fplot('3*x*sin(x) - 2*x', [-2*pi 2*pi], 'r')

This is working, But the command below is showing error.

fplot('diff(3*x*sin(x) - 2*x)', [-2*pi 2*pi], 'g')

Error Shown

??? Subscripted assignment dimension mismatch.

Error in ==> fplot at 105
x = xmin+minstep; y(2,:) = feval(fun,x,args{4:end});

Please give me some suggestions?

Answer 1

You could first calculate derivative, transform result in a string, and then plot obtained function:

syms x;
di = char( diff(3*x*sin(x)-2*x) );
fplot(di, [-2*pi 2*pi], 'g');

Edit: @shoelzer provides correct explanation for the error you’ve received.