cos x/3<√2/2 π/4+2πn<x/3<7π/4+2πn n∈z
3π/4+6πn<x<21π/4+6πn. n∈z
ответ x∈(3π/4+6πn. 21π/4+6πn) n∈z
A4-a2=0,4
a1+3d-(a1+d)=0,4
2d=0,4
d=0,4;2=0,2
S6=9
(2a1+5d)*6/2=9
(2a1+5*0,2)*3=9
2a1+1=9:3=3
2a1=3-1=2
a1=2:2
a1=1
A1 = - 3*1 - 3 = - 3 - 3 = - 6
a2 = - 3*2 - 3 = - 6 - 3 = - 9
d = a2 - a1 = - 9 + 6 = - 3