Đáp án: $VT=-\sin^4\alpha+\cos^4\alpha= \cos^4\alpha-\sin^4\alpha\\=(\cos^2\alpha+\sin^2\alpha)(\cos^2\alpha-\sin^2\alpha)\\=\cos2\alpha\\VP=2\cos^2\alpha-1=\cos2\alpha\Rightarrow đpcm$ Bình luận
Đáp án:
$VT=-\sin^4\alpha+\cos^4\alpha= \cos^4\alpha-\sin^4\alpha\\
=(\cos^2\alpha+\sin^2\alpha)(\cos^2\alpha-\sin^2\alpha)\\
=\cos2\alpha\\
VP=2\cos^2\alpha-1=\cos2\alpha\Rightarrow đpcm$