https://www.youtube.com/watch?v=wWGt30maKrY&list=PLohVThOFIb5cWHXGgHZ4aMPDlsuYaBslh&index=25
https://www.youtube.com/watch?v=yS_vmba3Rwo&list=PLohVThOFIb5cWHXGgHZ4aMPDlsuYaBslh&index=26
例題11
https://www.youtube.com/watch?v=k3ocMpo3cr4&list=PLohVThOFIb5cWHXGgHZ4aMPDlsuYaBslh&index=27