Question

пожалуйста!!! 
 cos(4x) + 5 cos^2(x) = 0,75 

Answer 1

cos²2x-sin²2x+5(cos2x+1)/2=0.75
2cos²2x-1+2.5cos2x+2.5=0.75
8cos²2x+10cos2x+3=0
d=100-96=4
cos2x=(-10+2)/16=-1/2    cos2x=(-10-2)/16=-3/4
2x=+-2π/3+2πk              2x=+-arccos(-3/4)+2πk
x=+-π/3+πk                    x=+-(1/2)arccos(-3/4)+πk