6cos^2(x)+11cos(x) +4=0

Question

6cos^2(x)+11cos(x) +4=0

Answer 1

6cos²x+11cos x+4 = 0
cos x = t, cos²x = t², t∈[-1; 1]
6t²+11t+4 = 0
D = 121-4*6*4 = 25
t1 = (-11-5)/12 = -1 1/3 - не подходит.
t2 = (-11+5)/12 = -1/2
cos x = 1/2
x = ±2π/3+2πn, n∈Z