Odpowiedź :
[tex]ctg\alpha=\frac{cos\alpha}{sin\alpha}[/tex]
[tex]\frac{sin\alpha*ctg\alpha}{cos\alpha}=\frac{sin\alpha*\frac{cos\alpha}{sin\alpha}}{cos\alpha}=\frac{cos\alpha}{cos\alpha}=1[/tex]
[tex]ctg\alpha=\frac{cos\alpha}{sin\alpha}[/tex]
[tex]\frac{sin\alpha*ctg\alpha}{cos\alpha}=\frac{sin\alpha*\frac{cos\alpha}{sin\alpha}}{cos\alpha}=\frac{cos\alpha}{cos\alpha}=1[/tex]