https://s3-us-west-2.amazonaws.com/secure.notion-static.com/49d75126-ad16-4cf1-a06d-da3d1945dd36/Untitled.png

要记住奇次根穿过，偶次根不穿过！！！！

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/be4d821b-356e-423d-9328-03338cb3721f/Untitled.png