-2cos2x=2-2cosx
-cos2x=1-cosx
-(2cos²x-1)=1-cosx
-2cos²x+1-1+cosx=0
2cos²x-cosx=0
2cosx(cosx-1/2)=0
cosx=0
x=π/2+πn, n∈Z
cosx=1/2
x=+-π/3+2πn n∈Z
ОТВЕТ -3π/2; -7π/3; -5π/3
a2=a1+d=17,
a6=a1+5d=65;
d=17-a1,
a1+5(17-a1)=65,
-4a1=-20,
a1=5,
S6=(a1+a6)6/2=(5+65)*3=210