@acer 

dear acer

thank you so much :) it was so simple and usefull :) i didn't know about this command at all :D

@Kitonum 

yes carl is right. i meant in case of exporting to matlab which one is better since i do not want to subs any value. by the way you aren't using matlab. thanks carl and kitonum :)

@Kitonum 

thank you so much kitonum

is there any different between f:= x(t)^2 + y(t)^2: and f:=t->x(t)^2+y(t)^2: since i'm gonna do just some parametric operations and no special value is gonna be calculated? (the result is gonna be exported to matlab)

which one is better for exporting?

(sry for mistakes due to bad engilsh)

thanks again :)

@Carl Love 

thank you so much carl