There is one type error on page 6, which is attached. The "l" should be "1". <img width="825" alt="屏幕快照 2019-11-21 上午11 33 27" src="https://user-images.githubusercontent.com/55707687/69301933-d5ada500-0c52-11ea-97c2-4c72e8fa69e2.png">
