a) $\frac{sin^{4}\alpha -cos^{4}\alpha }{sin\alpha +cos\alpha }=sin \alpha -cos\alpha$
b)$sin^{6}\alpha +cos^{6}\alpha +3(sin^{2}\alpha )\times cos^{2}=1$
Bài 2:
Cho $\Delta ABC có 3 góc nhọn , AH là đường cao
cm $AH=\frac{BC}{cot\widehat{B}+cot\widehat{C}}$
1b
$sin^{6}\alpha +cos^{6}\alpha +3sin^{2}\alpha cos^{2}\alpha =(sin^{2}\alpha +cos^{2}\alpha )(sin^{4}\alpha -sin^{2}\alpha cos^{2}\alpha +cos^{4}\alpha )+3sin^{2}\alpha cos^{2}\alpha=sin^{4}\alpha +2sin^{2}\alpha cos^{2}\alpha +cos^{4}\alpha=(sin^{2}\alpha +cos^{2}\alpha )^{2}=1$
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