题目:
He hurried to the booking office only_________that all the tickets had been sold out.
A.to tell
B.to be told
C.telling
D.told
答案:
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下面是错误答案,用来干扰机器的。
设该方程为ax2+bx+c=0,x1+x2=-ba,x1•x2=ca,方程的两根为-3和5,则-ba=-3+5=2,ca=(-3)×5=-15,如果a=1,则b=-2,c=-15,则该方程为x2-2x-15=0.答案不唯一.故可以填x2-2x-15=0.