URL：https://image.slidesharecdn.com/areasofcirclesandsectors-110411084046-phpapp01/95/areas-of-circles-and-sectors-7-728.jpg?cb=1302513344
掲載元：https://oer-studentresources.gesci.org/wp-content/courses/Maths/Maths-F3-Circles-Sector-and-Segments/lesson_development.html
