The Joy of Mathematics Season 1 Episode 9 The Joy of 9

  • TV-PG
  • April 11, 2007
  • 31 min

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.

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Watch The Joy of Mathematics - The Joy of 9 (s1 e9) Online - Watch online anytime: Buy, Rent
The Joy of Mathematics, Season 1 Episode 9, is available to watch and stream on The Great Courses Signature Collection. You can also buy, rent The Joy of Mathematics on demand at Apple TV Channels, Prime Video, Amazon, Kanopy, Hoopla online.
  • First Aired
    April 11, 2007
  • Content Rating
    TV-PG
  • Runtime
    31 min
  • Language
    English
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