Verified answer

2 + cos4x = 3cos2x

<=> 2 + 2cos²2x - 1 = 3cos2x

<=> 2cos²2x - 3cos2x + 1 = 0

=> cos2x = 1 => 2x = kπ => x= kπ/2

hoặc: cos2x = 1/2 = cosπ/3 => 2x = ± π + k2π => x = ± π/2 + kπ

su dung cong thuc ha bac cos4x

bien doi cos 2x

sau do lam theo tuan tu

dang thuc dk cm