Because 2a9 =3+a 12 (trick a 1+ an= a2+ an- 1, the sum is equal if the foot code is equal).

2(a 1+8d)= 3+a 1+ 1 1d。

A 1+5d=3, that is a6=3.

s 1 1 =( 1 1(a 1+a 1 1))/2

=( 1 1(a6+a6))/2

=( 1 1(3+3))/2

=33