スピログラフをいうパイポトロコイドを描く道具に関して考察を行なった。大円の中心からペン先までの距離が極小から次の極小へいくまでの間の曲線の部分に注目したとき,その部分が自己交差を起こすことを「逆行」と呼ぶ。この逆行を起こす条件は,大円の半径をa,小円の半径をb,小円の中心からペン先の距離をcとすると,a-b < c < b のとき,逆行が起こることを示した。
In Japan, geometry for 7th graders is treated as a preparation for formal (in a relative sense) geometry. But what is, or, what should be the preparation is a problem for which there can hardly be seen an effective answer in the long history of mathematics teaching. From the viewpoint of the histories of sciences, we can find the steps towards formalizations of their systems. These are 1) Gathering examples 2) Making a catalog or an order 3) Trial to find laws 4) Building its system. Formal geometry is for 4). We should focus on 1)-3) as the geometry for 7th graders. In this article, the concrete examples of class activities will be given.