# [Maxima] Question on ev

Andrei Zorine zoav1602 at gmail.com
Tue Nov 17 01:18:50 CST 2009

```There's another good function, called 'at'.

(%i1) x_sho : A*sin(w*t) + B*cos(w*t)\$
(%i2) diff(x_sho,t);
(%o2)                     w cos(t w) A - w sin(t w) B
(%i3) at(%,t=0);
(%o3)                                 w A

--
Andrei Zorine

>
> On Mon, Nov 16, 2009 at 2:23 PM, Conrad Schiff <gravmath at yahoo.com> wrote:
> > I need some help interpreting the behavior of the ev command.
> >
> > I enter the expression:
> > (%i1)  x_sho : A*sin(w*t) + B*cos(w*t)\$
> >
> > and then I want to setup equations for A and B in terms of x_sho(t=0) and vx_sho(t=0), where vx_sho = diff(x_sho,t).
> >
> > A straightforward application of ev leads immediately to one of the two equations I want:
> > (%i2)  IC1 : ev(x_sho,t=0) = x0;
> > (%o2)  A=x0
> >
> > however, my next attempt fails
> > (%i3)  IC2 : ev(diff(x_sho,t),t=0);
> > diff: second argument must be a variable; found 0
> >  -- an error.  To debug this try debugmode(true);
> >
> > I tried ev( ev(diff(x_sho,t),diff) , t=0 ), which also fails.  A trivial workaround is available by defining an intermediate expression vx_sho = diff(x,t) and then using ev(vx_sho,t=0) so this isn't a serious problem however it is annoying and the process looks so much cleaner if I don't have to define vx.
> >
> > Thanks,
> >
> >
> >
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> >
>
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```