8*(1-cos^2x)-5=2cosx
8cos^2x+2cosx-3=0
D=4-4*8*(-3)=4*(1+24)=4*25=100
cosx=-12/16 или сosx=8/16
cosx=-3/4
x= ± arccos(-3/4)+2Pin, n ∈ Z
x= ± (Pi- arccos(3/4))+2Pin, n ∈ Z
или
cosx=1/2
x= ± arccos(1/2)+2Pim, m ∈ Z
x= ± (Pi/3)+2Pim, m ∈ Z
О т в е т. ± (Pi- arccos(3/4))+2Pin, ± (Pi/3)+2Pim, m, n ∈ Z