In the first of what I hope to be a (nearly) weekly column, I’ll review GeoGebra, a free geometry software package.

GeoGebra is similar to Geometer’s Sketchpad in that you can perform all of the standard ruler-and-compass constructions (e.g., bisecting a segment, bisecting an angle), but you can also enter algebraic equations. So if you want to work with a circle, you can enter the equation directly, or you can create a point and use it as the center to draw a circle with the “circle” tool (GeoGebra will then find the equation for you, if you want). Here’s the construction of a regular pentagon in progress (click for full-size):

The interface is fairly intuitive. I didn’t need to refer to the documentation at all to do my first construction (the pentagon). In my attempt to construct the heptadecagon, round-off error caused the last side to be off slightly, and I haven’t tried again lately (because really, who wants to follow the instructions to construct a 17-sided polygon *twice*?), so I don’t know if that has been fixed.

GeoGebra is Java-based, so it’s a little slow to load (as are all Java apps), but you can download the whole thing and install it locally. It’s a nice thing to have when you’re not at school and can’t get to Geometer’s Sketchpad, and it generates very nice pictures if you need to include some in a document (LaTeX, Word, a blog).

As a big fan of free software (which will become more apparent the more I review), I like GeoGebra. If you ever need a little help with those geometry constructions, you can’t find a cheaper solution.