I want to put the result in a variable f.1(x), but the mathcad says that this variable is undefined. The only way that works is to assign by-hand (as seen below) but i would like to know about more clever solution
What you did should work and it actually does:
But without seeing your sheet its quite difficult to guess what could be wrong.
P.S.: I noticed the the result of the symbolic evaluation looks different in your pic (while being mathematically the same). Especially the factor 1 are somewhat irritating as Mathcad usually doesn't do this - even if you don't tell it to simplify.
Which version and maintenance release of Mathcad are you using?
MC15(M050) sheet attached
I tried a few illegal constructs, but didn't find the one that only marks (or appears to mark) the x argument red. Maybe 12 or 13 react differently from 11...
Guess we won't know until the OP attaches the worksheet.