Question

дана: ариф.прогрессия, S15=360. a13=9. a10/a6 - ?

Answer 1

a1+12d=9⇒a1=9-12d
(2a1+14d)*15/2=360
(12-24d+14d)*15=720
12-10d=720:15
12-10d=48
10d=12-48
10d=-36
d=-3,6
a1=9-12*(-3,6)=9+43,2=52,2
a10=a1+9d=52,2-9*3,6=19,8
a6=a1+5d=52,2-3,6*5=34,2
a10/a6=19,8:34,2=198/324=11/18