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It is divisible by 2 and by 9. [6] 342: it is divisible by 2 and by 9. 19: Add twice the last digit to the rest. (Works because (10a + b) × 2 − 19a = a + 2b; since 19 is a prime and 2 is coprime with 19, a + 2b is divisible by 19 if and only if 10a + b is.) 437: 43 + 7 × 2 = 57. Add 4 times the last two digits to the rest.
144, or twice 72, is also highly totient, as is 576, the square of 24. [8] While 17 different integers have a totient value of 72, the sum of Euler's totient function φ(x) over the first 15 integers is 72. [9] It also is a perfect indexed Harshad number in decimal (twenty-eighth), as it is divisible by the sum of its digits . [10]
Add up the numbers that make up the difference and the resultant number will always be evenly divisible by nine. For example, (72-27)/9 = 5. For example, (72-27)/9 = 5. Auditing transcription errors in medical research databases
The natural log of 2 is 0.693147, so when you solve for t using those natural logarithms, you get t = 0.693147/r.. The actual results aren’t round numbers and are closer to 69.3, but 72 easily ...
d() is the number of positive divisors of n, including 1 and n itself; σ() is the sum of the positive divisors of n, including 1 and n itselfs() is the sum of the proper divisors of n, including 1 but not n itself; that is, s(n) = σ(n) − n
Digit sums and digital roots can be used for quick divisibility tests: a natural number is divisible by 3 or 9 if and only if its digit sum (or digital root) is divisible by 3 or 9, respectively. For divisibility by 9, this test is called the rule of nines and is the basis of the casting out nines technique for checking calculations.
360 is divisible by the number of its divisors , and it is the smallest number divisible by every natural number from 1 to 10, except 7. Furthermore, one of the divisors of 360 is 72, which is the number of primes below it. 360 is the sum of twin primes (179 + 181) and the sum of four consecutive powers of three (9 + 27 + 81 + 243).
The digit sum of 2946, for example is 2 + 9 + 4 + 6 = 21. Since 21 = 2946 − 325 × 9, the effect of taking the digit sum of 2946 is to "cast out" 325 lots of 9 from it. If the digit 9 is ignored when summing the digits, the effect is to "cast out" one more 9 to give the result 12.