∵ U = ｛2,3,a ² + 2 a - 3｝,A = ｛a + 1 ,3｝
又∵ CUA = ｛5｝
∴ a ² + 2 a - 3 = 5 ①
a + 1 = 3 ②
由①得：
a ² + 2 a - 8 = 0
（a + 4）（a - 2）= 0
a + 4 = 0 或 a - 2 = 0
a = - 4 或 a = 2
由②得：
a = 3 - 1 = 2
综上,a = 2