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Topic: symbolic toolbox - diffentiating symbolic expressions
Replies: 2   Last Post: Oct 26, 2013 9:22 PM

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 CC Posts: 8 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..

Date Subject Author
10/26/13 CC
10/26/13 Nasser Abbasi
10/26/13 CC