Derive is a Home & Education software developed by bk teachware. After our trial and test, the software is proved to be official, secure and free. Here is the official description for Derive:

EditByMarydown: Derive 6 is a powerful system for doing symbolic and numeric mathematics on your personal computer. It processes algebraic variables, expressions, equations, functions, vectors, matrices and Boolean expressions like a scientific calculator processes numbers.

Problems in the fields of arithmetic, algebra, trigonometry, Calculus, Linear Algebra, and propositional calculus can be solved with the click of the mouse. Make plots of mathematical expressions in two and three dimensions using various coordinate systems. By its seamless integration of numeric, algebraic and graphic capabilities, Derive makes an excellent tool for learning, teaching and doing mathematics.

If you are familiar with Derive 5, you will find Derive 6 an (upward-compatible) extension. Here is a list of the major new features:
display the steps in the simplification of an expression along with the transformation rules applied
send and receive math worksheets to and from the TI-89, TI-92+, Voyage 200 TI CAS handhelds
animate parameterized expression plots with slider bars
automatically label plots showing the expression being plotted
rotate 3D plots using the mouse
easily navigate around the on-line help using the table of contents
customize menus, toolbars, and shortcut keys
profit from numerous other improvements, including
fully scaleable Derive Unicode font
support for Unicode characters and html Link hot spots in text objects
state variables saved in DfW files
optional multi-line editing
parentheses matching on the edit line
controllable display of 3D mesh lines and data-point sizes