1) \(\tan\alpha=\frac{\sin\alpha}{\cos\alpha}\)
bien doi ve phai ta co
\(\frac{\sin\alpha}{\cos\alpha}=\frac{doi}{huyen}:\frac{ke}{huyen}=\frac{doi}{huyen}.\frac{huyen}{ke}=\frac{doi}{ke}=\tan\alpha\)
2) \(\cot\alpha=\frac{\cos\alpha}{\sin\alpha}\)
bien doi ve phai ta co
\(\frac{\cos\alpha}{\sin\alpha}=\frac{ke}{huyen}:\frac{doi}{huyen}=\frac{ke}{huyen}.\frac{huyen}{doi}=\frac{ke}{doi}=\cot\alpha\)
3) \(\tan\alpha.\cos\alpha=1\)
\(\frac{\cos\alpha}{\sin\alpha}.\frac{\sin\alpha}{\cos\alpha}=1\)
4) \(\sin^2\alpha+\cos^2\alpha=1\)
\(\frac{doi^2}{huyen^2}+\frac{ke^2}{huyen^2}=\frac{huyen^2}{huyen^2}=1\)( su dung dinh ly pitago )
