Ramachandran, A.
(2019)
Generalising the divisibility tests of 9 and 11.
At Right Angles (5).
pp. 32-33.
ISSN 2582-1873
Abstract
Divisibility tests for 9 and 11 are generally taken up at upper
primary school level. The test for 9 seems to remain in
people’s consciousness long after they leave school and
move on to other things. The assertion that the sum of the digits of
any multiple of 9 is itself a multiple of 9 seems to give a mystical aura
to the number 9. The test for 11 is more involved and does not seem
to be retained as well, but other patterns in the multiples and powers
of 11 are widely appreciated.
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