I think 81 is composite and is 9 squared. 81 = 1 x 81, 3 x 27, or 9 x 9. Factors of 81: 1, 3, 9, 27, 81. Prime factorization: 81 = 3 x 3 x 3 x 3 which can also be written 3⁴. Since √81 = 9, a whole number, 81 is a perfect square. Even though it has other factors, the only multiplication fact we use is 9 x 9 = 81.
