What Maxima version did you use? With the 5.13.99rc2, I get -k/2 for both:
(%i9) subst(0,delta3,q);
(%o9) -k/2
(%i10) subst(0,delta3,trigsimp(q));
(%o10) -k/2
(%i11) build_info();
Maxima version: 5.13.99rc2
Maxima build date: 21:24 12/16/2007
host type: i686-pc-mingw32
lisp-implementation-type: GNU Common Lisp (GCL)
lisp-implementation-version: GCL 2.6.8
Barton
-----maxima-bounces at math.utexas.edu wrote: -----
>To: maxima at math.utexas.edu
>From: Constantine Frangos <cfrangos at telkomsa.net>
>Sent by: maxima-bounces at math.utexas.edu
>Date: 12/19/2007 12:46PM
>Subject: Maxima: trigsimp() introducing errors ??
>
>
>I have an expression q consisting of trigonometric terms, and q(delta3=0)
>=
>-k/2.
>
>When I apply trigsimp(q), the resulting expression has a singularity at
>delta3 = 0 (see below). It appears that trigsimp() is introducing errors
>??
>
>Any assistance would be appreciated.
>
>Regards,
>
>C. Frangos.
>
>
>(%i82) q;
>
>(%o82) -k*Lc/(Lc*(cos(delta3)*sqrt(sin(delta3)^2*Lc^2
> /(2*sin(delta3)*L2+cos(delta3)*Lc)^2
> +1)
> +1)
> +2*sin(delta3)*L2
>
>*sqrt(sin(delta3)^2*Lc^2/(2*sin(delta3)*L2+cos(delta3)*Lc)^2
> +1))
>(%i83) subst(0,delta3,q);
>
>(%o83) -k/2
>(%i84) subst(0,delta3,trigsimp(q));
>
>Division by 0
> -- an error. Quitting. To debug this try debugmode(true);
>(%i85)
>_______________________________________________
>Maxima mailing list
>Maxima at math.utexas.edu
>http://www.math.utexas.edu/mailman/listinfo/maxima