1 National Library Reference Alliance?
Choose this website to download free papers/books, and support the search of books, periodicals, papers, newspapers, audio and video. E-books are rich in resources, and downloaded files are collected in mailboxes.
2Mathtool？
At present, most of the functions of the website for learning necessary mathematical tools are free. Math Sketchpad can directly draw function diagrams, and also provides various auxiliary fixed-point line drawing tools, which support the export of multiple formats. A large number of math templates are more convenient to store in the cloud.
3Duolingo
Interesting websites for learning various languages, making courses based on basics, and combining games with learning.
4doyoudo？
Learning video clips and graphic design websites such as pr/ps/ae, the courses are rich in variety, high in quality, easy to understand, and the free courses are numerous and detailed, which is completely sufficient.
5 Large-scale open online courses in China University?
Websites containing many top university courses have been settled by major universities in China. There are a variety of courses, as well as practical courses such as CET-4 and CET-6, postgraduate entrance examination, public examination, final assault, etc., which are necessary for self-improvement.