S2/S6=[(a1+a2)*2/2]/[(a1+a6)*6/2]=1/3
(a1+a2)/[3(a1+a6)]=1/3
a1+a2=a1+a6
a2=a6
a6-a2=4d=0
d=0
所以是常数列
an=a1
S6/S12
=[(a1+a6)*6/2]/[(a1+a12)*12/2]
=(2a1*3)/(2a1*6)
=1/2
S2/S6=[(a1+a2)*2/2]/[(a1+a6)*6/2]=1/3
(a1+a2)/[3(a1+a6)]=1/3
a1+a2=a1+a6
a2=a6
a6-a2=4d=0
d=0
所以是常数列
an=a1
S6/S12
=[(a1+a6)*6/2]/[(a1+a12)*12/2]
=(2a1*3)/(2a1*6)
=1/2