– - 1. $\sin\alpha+\sin\beta=2\sin\displaystyle\frac{\alpha+\beta}{2}\cos\displaystyle\frac{\alpha-\beta}{2}$. 2. $\sin\alpha-\sin\beta=2\sin\displaystyle\frac{\alpha-\beta}{2}\cos\displaystyle\frac{\alpha+\beta}{2}$. 3. $\cos\alpha+\cos\beta=2\cos\displaystyle\frac{\alpha+\beta}{2}\cos\displaystyle\frac{\alpha-\beta}{2}$. 4. $\cos\alpha-\cos\beta=-2\sin\displaystyle\frac{\alpha+\beta}{2}\sin\displaystyle\frac{\alpha-\beta}{2}$. 5. Eger $\alpha\neq(2n+1)\dfrac{\pi}{2}$, $n\in Z$, $\beta\neq(2m+1)\dfrac{\pi}{2}$, $m\in Z$ bolsa, onda $\text{tg}\alpha+\text{tg}\beta=\displaystyle\frac{\sin(\alpha+\beta)}{\cos\alpha \cos\beta}$, $\text{tg}\alpha-\text{tg}\beta=\displaystyle\frac{\sin(\alpha-\beta)}{\cos\alpha \cos\beta}$. 6. Eger $\alpha\neq\pi n$, $n\in Z$, $\beta\neq\pi m$, $m\in Z$ bolsa, onda $\text{ctg}\alpha+\text{ctg}\beta=\displaystyle\frac{\sin(\alpha+\beta)}{\sin\alpha \sin\beta}$, $\text{ctg}\alpha-\text{ctg}\beta=\displaystyle\frac{\sin(\beta-\alpha)}{\sin\alpha \sin\beta}$.