Kosinussatz

$$\begin{array}{lcl} a^{2}=b^{2}+c^{2}-2bc\ \cos \alpha& \Le... ... & \cos \gamma =\frac{a^{2}+b^{2}-c^{2}}{2ab}\\ \end{array}$$

$$a^{2}+bc\ \cos \alpha =b^{2}+ca\ \cos \beta =c^{2}+ab\ \cos \gamma =\frac{a^{2}+b^{2}+c^{2}}{2}$$

Wenn $\alpha = 90^\circ$:

$$\begin{array}{lcl} \cos \beta =\frac{c}{a} & ; & \cos \gamma =\frac{b}{a}\\ \end{array}$$