The Joy of 9
Lecture no. 9 from the course: The Joy of Mathematics
Taught by Professor Arthur T. Benjamin | 31 min | Categories: The Great Courses Plus Online Mathematics Courses
Adding the digits of a multiple of 9 always gives a multiple of 9. For example: 9 x 4 = 36, and 3 + 6 = 9. In modular arithmetic, this property allows checking answers by "casting out nines." A related trick: mentally computing the day of the week for any date in history.