cos4θ=2cos22θ−1=2{2cos2θ−1}2−1=8cos4θ−8cos2θ+2−1\cos 4\theta = 2\cos^2 2\theta - 1 = 2\{2\cos^2\theta - 1\}^2 - 1 = 8\cos^4\theta - 8\cos^2\theta + 2 - 1cos4θ=2cos22θ−1=2{2cos2θ−1}2−1=8cos4θ−8cos2θ+2−1 =8cos4θ−8cos2θ+1=8x4−8x2+1= 8\cos^4\theta - 8\cos^2\theta + 1 = 8x^4 - 8x^2 + 1=8cos4θ−8cos2θ+1=8x4−8x2+1