`~rai~`
\(c)\sin\alpha-\sin\alpha\cos^2\alpha\\=\sin\alpha.(1-\cos^2\alpha)\\=\sin\alpha.\sin^2\alpha\\=\sin^3\alpha.\\e)\tan^2\alpha-\sin^2\alpha\tan^2\alpha\\=\tan^2\alpha.(1-\sin^2\alpha)\\=\dfrac{\sin^2\alpha}{\cos^2\alpha}.\cos^2\alpha\\=\sin^2\alpha.\\f)\cos^2\alpha+\tan^2\alpha\cos^2\alpha\\=\cos^2\alpha.(1+\tan^2\alpha)\\=\cos^2\alpha.\dfrac{1}{\cos^2\alpha}\\=1.\\\text{Giải thích:Công thức áp dụng:}\\+)\sin^2\alpha+\cos^2\alpha=1.\\+)\tan\alpha=\dfrac{\sin\alpha}{\cos\alpha}.\\+)\dfrac{1}{\cos^2\alpha}=1+\tan^2\alpha.\)
