{an} 的首项a,公差d
a1=a,a3=a+2d,a+a+2d=8,a+d=4 ...(1)
a2=a+d,a4=a+3d,a+d+a+3d=12,2a+4d=12,a+2d=6 .(2)
(2)-(1):d=2
代入(1):a=2
an=a1+(n-1)d
=2+(n-1)*2
=2n