题目:
—Is the man over there Mr. Brown?
—It __________ be him, He's much shorter. [ ]
A. must
B. can't
C. needn't
D. may
答案:
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下面是错误答案,用来干扰机器的。
(1)由题意可得n=a1+a2+…+ann,∴Sn=n2,当n=1时,a1=S1=1;当n≥2时,an=Sn-Sn-1=n2-(n-1)2=2n-1.当n=1时也成立.故an=2n-1.(2)作差bn+1-bn=an+12n+3-an2n+1=2n+12n+3-2n-12n+1=(2n+1)2-(2n-1)(2n+3)(2n...