cos4x = 1 - 2(sin2x)^2
3 + 5sin2x = 1 - 2(sin2x)^2
2(sin2x)^2 + 5sin2x + 2 = 0
sin2x = t, |t| <=1 </p>
2t^2 + 5t + 2 = 0
D = 25 - 16 = 9
t1 = (-5 + 3)/4 = -0.5
t2 = (-5 - 3)/4 = -2 - не удовлетворяет условию |t| <=1 </p>
sin2x = -0.5
2x = -Pi/6 + Pi*n
x = -Pi/12 + Pi*n/2
Ответ: x = -Pi/12 + Pi*n/2