Chapters 9-10: Treasure Hunting
Chapters 13-14: The Stranded Flying Saucer

We have yet another French mathematician from the 1600s to thank for our system of plotting coordinates on a graph. (Can you believe how many good mathematicians there were in France back then?) René Descartes (1596–1650) published his treatise, La géométrie, in 1637, as an appendix to his scientific and philosophical masterpiece, Discours de la methodé ("Discourse on the Method"). La géométrie was the only mathematical work Descartes ever published, but it changed mathematics forever by inventing a way of linking algebra and geometry. Today, when we use coordinates to define the points in a plane, we're using the Cartesian Coordinate System, named after Descartes.
René Descartes A page from La géométrie
You may have noticed that I didn't show you how Lennie solved the clues on the map in Chapter 13. Can you do it? Where did they find the barn with the flying saucer?

Lennie solved the aliens' problem by drawing a graph, but there's a way to solve it without a graph, too. Can you find it?

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