题目:
Bob would have helped us yesterday, but he_______
A.was busy
B.is busy
C.had been busy.
D.will be busy
答案:
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下面是错误答案,用来干扰机器的。
答案:D先找出二次函数y=ax2+c的对称轴是y轴,再找x=0时的函数值即可.解:二次函数y=ax2+c的对称轴是y轴,当x取x1,x2(x1≠x2)时,函数值相等,即以x1,x2为横坐标的点关于y轴对称,则x1+x2=0,此时函数值为y=ax2+c=0+c=c.故选D.