Question

14.4. Найдите п, если: n 2) a₁ = 5, d = 2, S = 192; n 3) a₁ = -12,5, d = 3, S = 195,5; n 4) a₁ = -2,4, d = -0,8, S = -70,4.

Answer 1

Объяснение:

Sn=2×5+2(n-1)/2×n=192

(10+2n-2)×n=192×2

8n×2n²=384

2n²+8n-384=0

n²+4n-192=0

D=4⁴-4×1×(-192)=784

1n=-4+28/2=12

2n=-4-28/2=-16

ответ: 12