问题
选择题
抛物线y=ax2+bx+c与x轴的公共点是（-1，0），（3，0），则这条抛物线的对称轴是直线（ ）
A．直线x=-1
B．直线x=0
C．直线x=1
D．直线x=3
答案
∵抛物线与x轴的交点为（-1，0），（3，0），
∴两交点关于抛物线的对称轴对称，
则此抛物线的对称轴是直线x=
=1．-1+3 2
故选C．
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抛物线y=ax2+bx+c与x轴的公共点是（-1，0），（3，0），则这条抛物线的对称轴是直线（ ）
A．直线x=-1
B．直线x=0
C．直线x=1
D．直线x=3
∵抛物线与x轴的交点为（-1，0），（3，0），
∴两交点关于抛物线的对称轴对称，
则此抛物线的对称轴是直线x=
=1．-1+3 2
故选C．
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