Как се намира cosa ,ако знаем cotga

[Гост]

Намерете cos[tex]\alpha[/tex] ,ако cotg\alpha =-2

[ptj]

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

[tex]cos^2\alpha=\frac{1}{1+tg^2\alpha}=\frac{1}{1+\frac{1}{cotg^2\alpha}}=\frac{cotg^2\alpha}{cotg^2\alpha+1}[/tex]