By far the most popular and well-stocked category of used instruments is our selection of guitars, where we've got a huge variety of all the big names. Used Gibson Les Paul, pre-owned Fender Telecaster and secondhand Stratocaster guitars are commonly found within these pages, waiting for their new homes with players and collectors.
22/7 is a pretty decent choice for teaching 11-year-olds about fractions that are close to pi. 355/113 would be better, if you think they can remember numbers that big. And approxRational is fun. Exercise for the interested reader: implement an inverse to approxRational.
1 You can measure Pi by bracketing a circle with polygons. 2  This is easy with polygons with small numbers of sides, but it gets harder as you add sides. 3  Archimedes used this trick with 96 sided polygons to correctly estimate Pi to about two digits (3.14), proving.