Question

Is 81 a prime number?

Answer 1

`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.