funktions(maths)

Question

Hej.
I'm trying to programm a program which shoud calculate funktions....
äm, i mean
the program shoud draw  the graf of the funktion and find "zero-points" and the extrem-points....

but have anyone any idears how i shoud make the "input"... so i mean how shoud the user enter the funktion?

thanks for idears (:
Lg Maxi