a4=9,a9=-6 => d=-( (9+6)/5) =-3a3=12;a2=15;a1=1818+15+12+9=54ответ:4 числа
9=a1+3d; -6=a1+8d; d=-3; a1=18; Sn=(2a1+(n-1)d)*n/2; 54=(2*18-3(n-1))*n/2; 54=(36+3-3n)*n/2; 108=(39-3n)*n; 108=39n-3n^2; 3n^2-39n+108=9; d= 225=15^2; n=4;
a4=9,a9=-6 => d=-( (9+6)/5) =-3
a3=12;a2=15;a1=18
18+15+12+9=54
ответ:4 числа
9=a1+3d; -6=a1+8d; d=-3; a1=18; Sn=(2a1+(n-1)d)*n/2; 54=(2*18-3(n-1))*n/2; 54=(36+3-3n)*n/2; 108=(39-3n)*n; 108=39n-3n^2; 3n^2-39n+108=9; d= 225=15^2; n=4;