Trigonometrinių funkcijų sumos bei skirtumo keitimas sandauga

1. \sin \alpha + \sin \alpha=2 \sin \dfrac{\alpha + \beta}{2} \cdot \cos \dfrac{\alpha - \beta}{2}

2. \sin \alpha - \sin \alpha=2 \sin \dfrac{\alpha - \beta}{2} \cdot \cos \dfrac{\alpha + \beta}{2}

3. \cos \alpha + \cos \beta=2 \cos \dfrac{\alpha + \beta}{2} \cdot \cos \dfrac{\alpha - \beta}{2}

4. \cos \alpha - \cos \beta=-2 \sin \dfrac{\alpha + \beta}{2} \cdot \sin \dfrac{\alpha - \beta}{2}

5. \rm{tg} \, \alpha + \rm{tg} \, \beta =\dfrac{\sin(\alpha + \beta)}{\cos \alpha \cdot \cos \beta}

6. \rm{tg} \, \alpha - \rm{tg} \, \beta =\dfrac{\sin(\alpha - \beta)}{\cos \alpha \cdot \cos \beta}

7. \rm{ctg} \, \alpha + \rm{ctg} \, \beta =\dfrac{\sin(\alpha + \beta)}{\sin \alpha \cdot \sin \beta}

8. \rm{ctg} \, \alpha - \rm{ctg} \, \beta =\dfrac{\sin(\alpha - \beta)}{\sin \alpha \cdot \sin \beta}