Giải thích các bước giải:
Bài 3
a) Ta có
$VT = \sin \alpha - \sin \alpha \cos^2 \alpha$
$= \sin \alpha(1 - \cos^2 \alpha)$
$= \sin \alpha . \sin^2 \alpha$
$= \sin^3 \alpha = VP$.
b) Ta có
$VT = \dfrac{1 + 2\sin \alpha \cos \alpha}{\cos^2 \alpha - \sin^2 \alpha}$
$= \dfrac{ \sin^2 \alpha + 2\sin \alpha \cos \alpha + \cos^2 \alpha}{(\cos \alpha - \sin \alpha)(\cos \alpha + \sin \alpha)}$
$= \dfrac{(\sin \alpha + \cos \alpha)^2}{(\cos \alpha - \sin \alpha)(\cos \alpha + \sin \alpha)}$
$= \dfrac{\sin \alpha + \cos \alpha}{\cos \alpha - \sin \alpha} = VP$.
