题目

若分式 的值恒为正数，求a的取值范围． 答案：解：∵ {#mathml#}3a−9a2−a−6{#/mathml#} = {#mathml#}3(a−3)(a−3)(a+2)=3a+2{#/mathml#} ， ∴要使分式 {#mathml#}3a−9a2−a−6{#/mathml#} 的值恒为正数，则a﹣3≠0，a+2＞0，∴a＞﹣2且a≠3He had been so absorbed____ watching the car passing by that he didn't notice his friends ____. A. to; coming       B. in; to come      C. on; coming       D. in; coming