/sci/ - Science & Math

I'm writing functions based on the compound interest formula in mathematica

on wolfram-alpha I can just type in a (mathematical) function and it'll output a graph for me.
How can I set a variable in mathematica such that I don't supply it, rather, mathematica generates a range of values based on changing that variable?

in other words, how do I make a programming variable behave as a mathematical variable in mathematica, I guess

the variable in question is shares_