a1=429 d=-22 Sn=3069 n-?

Sn=0,5*(a1+an)*n; 3069=0,5*(429+an)*n; 6138=(429+an)*n; an=a1+d(n-1)=429-22n+22=451-22n; 6138=(429+451-22n)*n; 6138=880n-22n^2; n^2-40n+279=0; D=1600-1116=484; n1=(40-22):2=9; n2=(40+22):2=31; ответ:9; 31