s3=1=（a1+a3)*(3/2)得a1+a3=2/3=a1+a1+d+a1+2d得a1+d=2/9=a2
sn=12=（a1+an）*n/2=(a1+an）*n=24
若n=12则a1+an=2=a1+a1+(n-1)d=a1+11d=2
得 d=8/45 a1=2/45故a3=16/45 s3=2/45+2/9+18/45=2/3
an=若n=12则a1+an=2=a1+a1+(n-1)d=2a1+11d=2
得 d=14/81 a1=4/81故an=?,an=?an-1?an-2=?代入,an+an-1+an-2不等3即舍