For Your Mathematics Teachers: Mathics.Net [Free Online Maths Resource]

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Link: Mathics

 

Login: Offered but not required to use the site.

 

Enter queries and submit them by pressing Shift + Return. See the gallery for some neat examples or the documentation for a full list of supported functions. Mathics uses MathJax to display beautiful math. Visit mathics.org for further information about the project.

 

Example plot:

 

http://i.snag.gy/6Ges6.jpg

 

Plotted expression:

 

Plot[{Sin[x], Cos[x], Tan[x]}, {x, -3Pi, 3Pi}]

 

Found on the Gallery page [link at the bottom of the first loaded page]