An - убывающая арифметическая прогрессия. а1 а3=-5; S3=6. Найдите S7.

А1*а3=-5;

S3=6, S7 - ?

a3=a1+2d

S3=2a1+2d/2 * 3 = (a1+d) * 3=3a1+3d;

{a1 (a1+2d) = - 5;

{3a1+3d=6;

{a1 (a1+2d) = - 5;

{a1+d=2;

{a1 (a1+2d) = - 5;

{d=2-a1;

a1 (a1+2 (2-a1)) = - 5;

a1 (4-a1) = - 5;

4a1-a1^2+5=0

a1^2-4a1-5=0;

a1=-1; a1=5;

d=2+1=3; d=2-5=-3;

S7 = (2 * (-1) + 6*3) / 2 * 7=56;

S7 = (2*5+6 * (-3)) / 2*7=-28.