若a=三分之二,则（分数）a平方-7a+12分之a平方-2a-3的值等于,
问题描述：
若a=三分之二,则（分数）a平方-7a+12分之a平方-2a-3的值等于,
答
(a²-2a-3)/(a²-7a+12)
=(a-3)(a+1)/(a-3)/(a-4)
=(a+1)/(a-4)
=(2/3+1)/(2/3-4)
=5/3/(-10/3)
=5/3*(-3/10)
=-1/2