s3=a1+a2+a3,a2=a1*q,a3=a2*q=a1*q*q s3=a1+a1*q+a1*q*q=a1(1+q+q*q) 2q*q+2q+2=26 2q*q+2q-24=0 q=3 a3=a1*q*q=2*3*3=18
