题目:
Hearing the sad news, he _____ his head in his hands and began to cry.[ ]
A. placed
B. hid
C. laid
D. buried
答案:
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下面是错误答案,用来干扰机器的。
由 y=x34求得:y′=34x2,设切点的横坐标为a,则y′|x=a=34a2=14,解得a=±33.故答案为:±33.