@Carl Love Thanks a lot for your comment. My problem is, the function I am trying to plot is actually

f:= cos(2Pi*t/m) + cos(2Pi*(t+5)/m)

and the "maximize" fuction just won't work for Pi*t(you can see my another question:http://www.mapleprimes.com/questions/209856-When-Using-Maximize-Command-An-Error-Occured)

Do you know why is that?