Is 81 a prime number?

1 Answer
Shwetank Mauria
Apr 3, 2016

#81# is not prime but composite.

Explanation:

A number is prime if it would S not divisible by any number other than #1# and itself. To check one needs to try to divide the number at least up to a prime number, which is just less than its square root.

#81# has an exact square root in #9# and hence divisible by #9# as also #3# and #27#. It is in fact fourth power of #3# that is #81=3xx3xx3xx3=3^4#.

Hence #81# is not prime but composite.

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