Production of teaching AIDS in circular areas: 1. Replace the board (not too thick) with cardboard. 2. Make a circle and divide it into 15 equal parts along the radius. 3. Take one of them and divide it into two parts equally along the radius of the circle. So far, there are 14 whole copies and 2 half copies. 4. Take two pieces of cloth and stick seven whole pieces to one of them along the circumference, and then stick 1 half piece. 5. Repeat the above operation for another piece of cloth. 6. Cut off the remaining cloth.