5cos^2(x)+12cos(x)+7=0

Question

5cos^2(x)+12cos(x)+7=0

Answer 1

5cos²x+12cos x+7 = 0
cos x = t, cos²x = t², t∈[-1; 1]
5t²+12t+7 = 0
D = 144-4*5*7 = 4
t1 = (-12-2)/10 = -1,4  - не подходит
t2 = (-12+2)/10 = -1
cos x = -1
x = π+2πn, n∈Z