3 cos^2 - 5 cos x - 12 = 0

Question

3 cos^2 - 5 cos x - 12 = 0

Answer 1

Cosх=у
3у-5у-12=0
D =(-5)^2-4*3*(-12)=25+144=169
х=(5-13)/2*3=-8/6=-4/3
х=(5+13)/2*3=18/6=3