Áp dụng $\sin^2\alpha + \cos^2\alpha = 1$ :
$\sin^6 \alpha + \cos^6 \alpha + 3 \sin^2 \alpha \cos^2 \alpha = \sin^6 \alpha + \cos^6 \alpha + 3 \sin^2 \alpha \cos^2 \alpha ( \sin^2 \alpha + \cos^2 \alpha) = (\sin^2 \alpha + \cos^2\alpha)^3 = 1^3 = 1$