Lesson 26. According to the information on the website, in the 26th class of Zhang Yu's graduate mathematics elective course, Zhang Yu explained Cramer's rule, that is, if the determinant of the coefficient matrix of a linear equation group is not equal to zero, then the linear equation group has a unique solution. Zhang Yu, the main lecturer of this course, is a well-known expert in mathematics counseling for postgraduate entrance examination in China and a "backbone teacher of national excellent curriculum construction" by the Ministry of Education.