Question

А3=-8; а5=4; S10-? Арифметическая прогрессия помогите пожалуйста

Answer 1

S10 = (a1 + a10)*10/2 = 5(a1 + a1 + 9d) = 5(2a1 + 9d)

a3 + a5 = a1 + 2d + a1 + 4d = 2a1 +6d = -4

a1 + 3d = -2

a1 + 2d = -8

d = 6

a1 = -8 - 12 = -20

S10 = 5(-40 + 54) = 14*5 = 70