I had just started back for the house to change my clothes ___ I heard voices. A.as B.when C.after D.while 答案:B如图,二次函数y=ax2+bx+c(a≠0)的图象的顶点C的坐标为(﹣1,﹣3),与x轴交于A(﹣3,0)、B(1,0),根据图象回答下列问题:(1)写出方程ax2+bx+c=0的根;(2)写出不等式ax2+bx+c>0的解集;(3)若方程ax2+bx+c=k有实数根,写出实数k的取值范围.