a6=a1+5d=3,
a9=a1+8d=18,
a1=3-5d,
3-5d+8d=18,
3d=15,
d=5.
a6=a1+5d; 3=a1+5d; a1=3-5d;
a9=a1+8d; 18=a1+8d; 18=3-5d+8d; 15=3d; d=5;
разность прогрессии = 5
a6=a1+5d=3,
a9=a1+8d=18,
a1=3-5d,
3-5d+8d=18,
3d=15,
d=5.
a6=a1+5d; 3=a1+5d; a1=3-5d;
a9=a1+8d; 18=a1+8d; 18=3-5d+8d; 15=3d; d=5;
разность прогрессии = 5