Question

Помогите пожалуйста.
Арифметическая прогрессия

Answer 1

a2+a6=26 => 2a1+6d=26 => a1+3d=13 => a4 = 13
a2=a1+d=9
a4=a1+3d=13
Значит, 2d=4 => d = 2
a1=a2-d=9-2=7
S6 = (a1+a6)*6/2 = 3 (7+17) = 3 * 24 = 72
a7 = a1+6d = 7+6*2=7+12=19
Ответ: 19, 13, 2, 72
3) a5+a9=6 => 2a1+12d=6 => a1+6d=3 => a7 = 3
a1+6d=3
a1+4d=9
2d=-6 = > d = -3
a1+4d=9 => a1=9-4d = 9+12=21
a4=a1+3d=21+3*(-3) = 21-9=12
a8 = a9 - d = -3 - (-3) = 0
S8 = (a1+a8) * 8 / 2 = a1 * 4 = 21 * 4 = 84
Ответ: 3, 12, -3, 84