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Taught by Professor Arthur T. Benjamin | 32 min | Categories: Mathematics
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.
24 Lectures
1
The Joy of Math: The Big Picture
0
of 30 min
2
The Joy of Numbers
0
of 29 min
3
The Joy of Primes
0
of 30 min
4
The Joy of Counting
0
of 28 min
5
The Joy of Fibonacci Numbers
0
of 30 min
6
The Joy of Algebra
0
of 31 min
7
The Joy of Higher Algebra
0
of 31 min
8
The Joy of Algebra Made Visual
0
of 32 min
9
The Joy of 9
0
of 32 min
10
The Joy of Proofs
0
of 31 min
11
The Joy of Geometry
0
of 31 min
12
The Joy of Pi
0
of 31 min
13
The Joy of Trigonometry
0
of 32 min
14
The Joy of the Imaginary Number i
0
of 31 min
15
The Joy of the Number e
0
of 30 min
16
The Joy of Infinity
0
of 30 min
17
The Joy of Infinite Series
0
of 31 min
18
The Joy of Differential Calculus
0
of 32 min
19
The Joy of Approximating with Calculus
0
of 31 min
20
The Joy of Integral Calculus
0
of 32 min
21
The Joy of Pascal’s Triangle
0
of 31 min
22
The Joy of Probability
0
of 30 min
23
The Joy of Mathematical Games
0
of 31 min
24
The Joy of Mathematical Magic
0
of 31 min
