Quote:
[MapleSoft's] core intellectual property was developed as an advanced research project [...] in the early 1980s.
(MapleSoft Website)
So Maple (now in its 13th release) is the product of almost 30 years of work.
A simple calculator with support for variables isn't a huge undertaking. One that supports functions, integration, etc... to a usable degree of robustness is. Start by developing a way to model mathematical (as opposed to programmatic) functions in a way that allows you to inspect and act on them, then start creating procedures that do so.