a3=7
a9=18
a3=a1+2d
a9=a1+8d
{a1+2d=7
{a1+8d=18
6d=11
d=11/6
a1+2*(11/6)=7
a1+11/3=7
a1=7-11/3=21/3-11/3=10/3=3 1/3=3,(3)
a6=a1+5d=10/3+5*(11/6)=20/6+55/6=75/6=25/2=12,5
ответ: 3 1/3; 12,5
