因为,分式（3a - 2)/(2a^2 - 1)的值为0,即(3a - 2)/(2a^2 - 1) = 0
所以有, 3a - 2 = 0
解得：a = 2/3
因此, (a^2 - 2a - 3)/(a^2 - 7a + 12) = ( a - 3)(a + 1)/( a - 3)(a - 4)
= (a + 1)/( a - 4)
把 a = 2/3代入上式得：
（a + 1)/(a - 4)= ( 2/3 + 1)/( 2/3 - 4)
= (5/3 ) / ( - 10/3)
= 5/3 × （- 3/10)
= - 1/2