등차 수열 a1+a3=4,a4+a6=12,a8+a10=?

등차 수열 a1+a3=4,a4+a6=12,a8+a10=?

a1+a3=4
a4+a6=12
3d+3d=8
6d=8
d=4/3
a8+a10=a1+7d+a3+7d=a1+a3+14d=4+14d
=4+14*4/3
=68/3