A3+a7=24 a3×a7=128 найти d; a1; a5; s8 - вопрос №5152518372437128158 от Nikstone 19.09.2022 06:13

Question

Алгебра: A3+a7=24 a3×a7=128 найти d; a1; a5; s8

Nikstone · Answer

A1+2d+a1+6d=24⇒2a1+8d=24⇒a1+4d=12⇒a1=12-4d
(a1+2d)(a1+6d)=128
(12-4d+2d)(12-4d+6d)=128
(12-2d)(12+2d)=128
144-4d²=128
4d²=144-128
4d²=16
d²=4
1)d1=-2
a1=12+8=20
a5=a1+4d=12
S8=(2a1+7d)*8/2
S8=(40-14)*4=26*4=104
2)d=2
a1=12-8=4
S8=(8+14)*4=22*4=88