У"+5у'=0
у"+6у'+13у=0
у"=-5у'
-5у'+6у'+13у=0
у'+13у=0
А1+10d–a1–2d–a1–7d=27
a1+5d+a1+13d=86
d–a1=27
2a1+18d=86 |:2
a1=d–27
a1+9d=43
d–27+9d=43
10d=70
d=7
a1=7–27=–20
2a1+17d
S18 = ------------ • 18 = (2•(–20)+17•7)9=
2
= (–40+119)•9 = 711
Log7(5x-4)>=0
ОДЗ:
5x-4>0
5x>4
x>0,8
Далее:
log7(5x-4)>= log7(1)
5x-4>=1
5x>=1+4
5x>=5
x>=5
Ответ:[5; + бесконечность)