Sn=(2a1+d(n-1))/2*n
S3=(2a1+2d)/2*3
S3=3a1+3d
36=3a1+3d
3a1=36-3d
a1=12-d
a2=a1+d - по правилу an= an-1+d
a2=12 => AC=12
Sn=(2a1+d(n-1))/2*n
S3=(2a1+2d)/2*3
S3=3a1+3d
36=3a1+3d
3a1=36-3d
a1=12-d
a2=a1+d - по правилу an= an-1+d
a2=12 => AC=12