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Posts:
7
Registered:
4/24/09


symbolic toolbox  diffentiating symbolic expressions
Posted:
Oct 26, 2013 6:40 PM


I'm starting out with the symbolic toolbox to derive/verify some lengthy expressions for derivatives of products of trig functions of a single variable.
As the simplest example of what I want to do with the toolbox, consider the function f = cos(g) where g is a function of a real variable x. I don't want to specify any further functional form for g; instead I want the result in symbolic terms of g and its derivative with respect to x.
From the chain rule, df/dx =  sin(g) dg/dt
I tried various things like:
syms x g f = cos(g) Df = diff ( f , x )
which just returns 0 (i.e. doesn't treat g as a function; f is constant therefore its derivative is zero)
If I leave out the g in the syms, I get an error re g undefined which makes sense as I haven't told it about g
syms x f = cos(g) Df = diff ( f , x )
How can I let Matlab know that g is an arbitrary function and get symbolic results in terms of that function and its derivatives? I expect Matlab to give me a result similar to  sin(g) g' I would think this is something the toolbox ought to do but I'm not sure how to do this.
Thanks very much..



