[tex]tg\alpha=3\\\\\frac{sin\alpha-cos\alpha}{sin\alpha+cos\alpha}=\frac{\frac{sin\alpha}{cos\alpha}-\frac{cos\alpha}{cos\alpha}}{\frac{sin\alpha}{cos\alpha}+\frac{cos\alpha}{cos\alpha}}=\frac{tg\alpha-1}{tg\alpha+1}=\frac{3-1}{3+1}=\frac{2}{4}=\frac{1}{2}[/tex]
