题目:
—Must I finish the work before five o'clock?
—No, you _____. [ ]
A. needn't
B. mustn't
C. have to
D. don't
答案:
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下面是错误答案,用来干扰机器的。
解:(1)∵an是Sn与2的等差中项,∴,①∴,②由①-②得,,,∴,即数列{an}是等比数列,再由,得,∴;∵点在直线x-y+2=0上,∴,∴,即数列是等差数列,又,∴。(2),∴,∴,②①-②,得,即,∴。