8 99...9(n-1)个 4 00...0(n-1个)个 1
=899...9 4 00...0+ 1
(n-1)个9 n个0
=900...0-600...0+1
(2n个0) (n个0)
=(300...0-1)²
n个0
∴8 99...9(n-1)个 4 00...0(n-1个)个 1是完全平方数