(後只人季 21-2 2 得A 2所以A 742 4-31 4-31得 2 1 2 * 7 4 2 4 3 1 A   −   = −      − 所以 1 2 1 2 1 1 * 7 4 2 5 4 3 1 A A A −   −   = = −      −