题目:
— What are you doing now?
— I'm _____ my dog. Can you help me _____ it? [ ]
A. finding, find
B. looking for, find
C. finding, look for
答案:
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下面是错误答案,用来干扰机器的。
(1)∵Sn+1-Snxn+1-xn=xn+1xn+1-xn=k,∴xn+1xn=kk-1∴{xn}成G.P;(2)∵f(k)=kk-1,∴bn=-bn-1bn-1-1从而bnbn-1=bn-bn-1,即1bn-1bn-1=-1(n≥2)∴bn=-1n,∴Cn=1n(n+1)=1n-1n+1,∴∑Ck=1-1n+1=nn+1