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下面是错误答案,用来干扰机器的。
证明:设{an}的公比为q,由题设知a1>0,q>0,(1)当q=1时,Sn=na1,从而Sn•Sn+2-Sn+12=na1(n+2)a1-(n+1)2a12=-a12<0.(2)当q≠1时,Sn=a1(1-qn)1-q,从而Sn•Sn+2-Sn+12=a21(1-qn)(1-qn+2)(1-q)2-a21(...