update lesson7

Author: yizhen20133868

README.md

@@ -43,3 +43,8 @@
 * [知乎专栏笔记地址](https://zhuanlan.zhihu.com/p/105280523)
 * [PDF下载地址](https://github.com/yizhen20133868/MIT-Linear-Algebra-Notes/tree/master/%5B06%5D%20%E5%88%97%E7%A9%BA%E9%97%B4%E5%92%8C%E9%9B%B6%E7%A9%BA%E9%97%B4)
 
+第七课：求解 Ax = 0，主变量，特解
+* [中文字幕课程地址](http://open.163.com/newview/movie/free?pid=M6V0BQC4M&mid=M6V29FRJK)
+* [知乎专栏笔记地址](https://zhuanlan.zhihu.com/p/106145156)
+* [PDF下载地址](https://github.com/yizhen20133868/MIT-Linear-Algebra-Notes/tree/master/%5B06%5D%20%E5%88%97%E7%A9%BA%E9%97%B4%E5%92%8C%E9%9B%B6%E7%A9%BA%E9%97%B4)
+