a5=a1+4d=8S5=(a1+a5)*5/2=5a1+10d=20d=2a1=0an=2*(n-1)Sn=(a1+an)*n/2=(n-1)*n(2)令bn=Sn+2an+1/6=n^2+3n-4+1/6b1=1/6b2=6+1/6>12/25所以t最小值为1
