a1+a2+a3=12
a1a2a3=48
2a2=a1+a3
a2=4
a1=a2-d=4-d
a3=a2+d=4+d
4(4-d)(4+d)=48
d=±2
a1=6或a1=2
数列{an}的通向公式
an=6-2(n-1)
=8-2n
或an=2+2(n-1)
=2n