TITLE: How to tell Mathematica about a constant?
QUESTION [7 upvotes]: Sometimes I want to ask mathematica to plot or calculate something without given a certain value for it ..
For example I may want to ask mathematica to integrate $\int{a x} \mathrm{d}x$! How to tell him that $a$ is a constant?

REPLY [5 votes]: The previous answers provide enough context I believe. It is sometimes also useful to give mathematica information about the type of constants (if it's a real or imaginary number for example). This can be done using the option Assumptions. E.g. Integrate[Exp[a x],{x,0,Infinity},Assumptions->{a<0}]