It is a square number because 9 times 9 equals 81
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Assuming you mean the arithmetic mean, it is 80.
81 is not prime it's a composite number
Eighth letter is :H. First is one= lettera
It isn't. The square of any real number is positive. (negative 9)2 = positive 81. Did you mean negative (92) ? Well, 92 = 81, and the negative of that is negative 81.
81 is not a prime number. 81 = 3 * 3 * 3 * 3