pt <=> cos4x - 2cos2x + 1 - 2cos22x + 1 = 0
<=> cos4x - 2cos2x + 1 - 2(2cos2x - 1)2 + 1 = 0
<=> cos4x - 2cos2x +1 - 8cos4x + 8cos2x - 2 + 1 = 0
<=> -7cos4x + 6cos2x = 0 ..............
srr cậu mh ghi đề sai rồi .. giải lại hộ mh vs $4cos^{4}x-cos2x-cos4x$