@ @pbjïØ Üa@ @RT . [Aust.MC 1981] ( ) 87 4 0, 1, 2, …, 9 . . . ABC . 3 .8 7 4 . 3 × 3 × 3 = 27 28 [Aust.MC 1981] ( ) . n −1 .n . n(n − 1) .n(n − 1) = 56 . n(n − 1) = 28 2 n2 − n − 56 = 0 2 .n = 8 (n + 7)(n − 8) = 0 n .n = −7 n = 8