GeoGebra is a free suite of software which can be used to explore mathematics. It is particularly powerful as it makes it easy to link visual geometry with the algebraic representations of things- hence the hybrid name Geo from geometry and Gebra from algebra. It is easy for learners to use to use. The software is easy to download but also can be used in an internet without downloading it. To get started just go to the GeoGebra Homepage http://www.geogebra.org/cms/.
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When you have loaded GeoGebra, you will get the GeoGebra Drawing Pad in its own window, at the bottom of the screen is the Input Line, and on the left of the screen is the Algebra View. Now you will be ready to explore.
There is also a list of useful websites for GeoGebra, an excellent short article on using GeoGebra with still photos by Prof A Oldknow, some screencast videos on how to use GeoGebra to examine photos, some photos to practice with and some GeoGebra (.ggb) files with photos pre-loaded for you to practice with.