若两条直线ax+2y+6=0与x+（a-1）y+（a2-1）=0平行，则a的取值集合是_．

∵两条直线ax+2y+6=0与x+（a-1）y+（a2-1）=0平行，直线ax+2y+6=0的斜率存在，∴x+（a-1）y+（a2-1）=0斜率也存在，两条直线ax+2y+6=0与x+（a-1）y+（a2-1）=0分别化为：y＝−a2x−3，y＝−1a−1x−(a+1)．∴−a2＝...