VT = cos 4x = cos(2.2x) = 2.cos^2 2x - 1 = 2(2cos^2 x - 1)^2 - 1 = 2(4.cos^4 x - 4.cos^2 x + 1) - 1 = 8.cos^4 x - 8.cos^2 x + 2 - 1 = 8cos^4x - 8cos^2x + 1 = VP
$VT = cos 4x = cos(2.2x) = 2.cos^2 2x - 1 = 2(2cos^2 x - 1)^2 - 1
$$= 2(4.cos^4 x - 4.cos^2 x + 1) - 1 = 8.cos^4 x - 8.cos^2 x + 2 - 1
$$= 8cos^4x - 8cos^2x + 1 = VP
$