Answer :
[tex]8 {sin}^{2} x + cos \: x - 5 = 0[/tex]
[tex]recall \: that \: {sin}^{2} x + {cos}^{2} x = 1[/tex]
[tex]then \: {sin}^{2} x = 1 - {cos}^{2} x[/tex]
then substitute,
[tex]8( 1 - {cos}^{2} x) + cos \: x - 5 = 0[/tex]
After Further Simplication,
[tex]8 {cos}^{2} x - cos \: x - 3 = 0[/tex]
[tex]let \: y = \cos(x) [/tex]
[tex]8 {y}^{2} - y - 3 = 0[/tex]
use quadratic formulae
[tex]y = 0.375 \: or \: - 0.25[/tex]
therefore
[tex] \cos(x) = 0.375 \: or \: - 0.25[/tex]
[tex] x = 70degrees \: or \: 104.5degrees[/tex]