Derivative and Mathcad Prime

The disappointing answer is that you can't. "sec" is not among the allowed modifiers for "rewrite" (https://support.ptc.com/help/mathcad/r6.0/en/index.html#page/PTC_Mathcad_Help%2Frewriting_expressions.html%23) and i see no other way to force the symbolics to use sec.

The best you could do is to use "rewrite,cos" and then conclude yourself what the result would be in terms of "sec".

Generally its hard to impossible to force the output of a symbolic evaluation in Mathcad to be displayed in a specific way, neither the functions used nor the order of summands and variables. In contrast to other programs, we have far too few opportunities to intervene in a controlling manner.

EDIT: One should never say that something can't be done 🙂

Here is a way to achieve what you are looking for but its quite awkward and its doubtful if it would work as flawless for more complicated functions, too.