Substitution of dependend variable in differentiation

Marc Hodapp wrote:

> Hi everybody,
>
> I have got a function u depending on x which I differentiate with 
> respect to x.  I then want to substitute  u(x)=l*U(x),  in order to 
> get  du/dx = l*dU/dx.  What I have done so far is :
>
> (%1)       expr1:diff (u(x),x)
> (%2)       expr2:subst(l*U(x),u(x),expr1)
> (%o2)     d(l*U(x)) / dx
>
> How can I obtain my desired result? 


I think you can just do this:
%,diff   or  ..

ev(%o2,diff);

e.g.

%i10) %,diff;
                    d
(%o10)           L (-- (U(x)))
                    dx
(%i11)


>
>
> Many thanks in advance.
>
> Marc Hodapp
>
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