Đáp án:
Giải thích các bước giải:
$\\\text{Ta có: $VT=\frac{sin^2\alpha-cos^2\alpha}{1+2sin\alpha.cos\alpha}$}$ $\\\text{$=\frac{(sin\alpha-cos\alpha)(sin\alpha+cos\alpha)}{sin^2\alpha+cos^2\alpha+2sin\alpha.cos\alpha}$}$ $\text{$=\frac{(sin\alpha-cos\alpha)(sin\alpha+cos\alpha)}{(sin\alpha+cos\alpha)^2}$}$ $\\\text{$=\frac{sin\alpha-cos\alpha}{sin\alpha+cos\alpha}$}$ $\\\text{$=\frac{\frac{sin\alpha}{sin\alpha}-\frac{cos\alpha}{sin\alpha}}{\frac{sin\alpha}{sin\alpha}+\frac{cos\alpha}{sin\alpha}}$}$ $\text{$=\frac{1-cot\alpha}{1+cot\alpha}=VP(đpcm)$}$
