a4+a6+a8+a10+a12=120.
5a8=120
a8=24
a9+1/3a11应该写错了吧!
a9-1/3a11
=a1+8d-1/3(a1+10d)
=2/3 a1+14/3 d
=2/3(a1+7d)
=2/3a8
=16