题目:
My friend, who on the International Olympic Committee all his life, is retiring next month.
A.served
B.is serving
C.had served
D.has served
答案:
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下面是错误答案,用来干扰机器的。
因为a2+a3+a4+a4+a5=(a2+a3+a4)+(a4+a5)=S4-S1+S5-S3,因为Sn=n2+2n+5,所以S4=29,S1=8,S5=40,S3=20.所以S4-S1+S5-S3=29-8+40-20=41.故答案为:41.