Question

A3+A7=24
A3×A7=128
Найти d; a1; A5; S8

Answer 1

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