2Sn=an^2+n-4 2Sn-1=(an-i)^2+n-5 2an=2Sn-2Sn-1=an^2+n-4 -((an-i)^2+n-5)
= an2+n-4-(an-1)^ 2-n+5=an2-(an-1)^ 2+1 an^2-2an+1=(an-1)^2 ( an -1)^2=(an-1)^2
an -1=an-1 an-an-1=1 首项2a1=a1^2+1-4 a1=3 数列是手下那个为3 ，公差为1的等差。an=a1+(n-1)d=3+(n-1)1=n-2
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