Re: [Maxima-discuss] Determining Real Part

[Thomas D. D. <to...@wa...>]

On 02/16/14 03:15, Rupert Swarbrick wrote:

> Ahah! The first problem is that Maxima doesn't know p(t) is real. If it
> did, then all those horrible atan2's would disappear. Let's try again:
>
>    (%i16) declare(p, real);

Thank you, this is the key.  I could not discover how to tell Maxima 
that p(t) was real.  I was misleading in my subject.  I wanted to get 
the real solution from the solution set.

kill(all);
p_val:40;  /* dynes/cm^2 */
dp_val:3;  /* dynes/cm^2/second */
v_val:60;  /* cm^3 */
assume(p(t)>0);
declare(p,real);
soln_c:solve(p_val*v_val^(14/10)=c,c);
eqn:p(t)*v(t)^(14/10)=rhs(first(soln_c));
for s in solve(eqn,v(t)) do (
   tmp:rhs(s),
   if realpart(tmp) = tmp then
   soln_vt:tmp
   )$
define(v(t),soln_vt);
define(dv(t),diff(v(t),t));
subs:[diff(p(t),t)=dp_val,p(t)=p_val];
dv:subst(subs,dv(t));

Tom Dean