about account add arrow-down arrow-left arrow-right arrow-up back-arrow register-arrow book-mobile book categories chat-bubble-mobile chat-bubble close college contact-us credit-card drag email-square facebook-mobile facebook-square facebook faq film history home load modal-error person pinterest-square play-mobile play queue remove resume search share show star tick trailer trash twitter-mobile twitter-square twitter university warning warning youtube-square open-eye close-eye promo-tag tag check info info active
9

The Joy of 9

Lecture no. 9 from the course: The Joy of Mathematics

The Joy of 9

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.

Reviews

c********u
October 1, 2017

Reply

Delete

p********m
December 11, 2016

Reply

Delete