7cosквадрат х -cosx- 8

[latex]7cos ^{2} x-cosx-8 \ \ 7cos ^{2} x-cosx-8=0 \ cosx=t \ 7t^{2} -t-8=0 \ D=1+224=225 \ sqrt{D} =15 \ t _{1} = frac{1+15}{14} = frac{16}{14} = frac{8}{7} \ t _{2} = frac{1-15}{14} = frac{-14}{14} =-1 \ cosx eq frac{8}{7} \ cosx=-1 \ x= pi +2 pi n [/latex]n∈Z