Объяснение:
1)
= 387
2) a₁ = 12.5
a₂ = 11.2
d = a₂-a₁ = -1.3
a₁ +d(n-1) <0
12.5 - 1.3(n-1) <0
-1.3n +13.8 <0
n > 138/13
n = 11
Объяснение:
1)![S_{n} = \frac{2a_{1}+d(n-1)}{2} *n](/tpl/images/1561/8016/1ba06.png)
2) a₁ = 12.5
a₂ = 11.2
d = a₂-a₁ = -1.3
12.5 - 1.3(n-1) <0
-1.3n +13.8 <0
n > 138/13
n = 11