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$ a = \sqrt{b^{2} +c^{2} -2\cdot b\cdot c\cdot cos\alpha } $
$ cos\alpha = \frac{b^{2} +c^{2} -a^{2} }{2\cdot b\cdot c} $
Geometrie-Trigonometrie-Kosinussatz
$ a^2 = b^{2} +c^{2} -2\cdot b\cdot c\cdot cos\alpha $
$a = \sqrt{b^{2} +c^{2} -2\cdot b\cdot c\cdot cos\alpha }$
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$cos\alpha = \frac{b^{2} +c^{2} -a^{2} }{2\cdot b\cdot c}$
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Beispiel Nr: 03
$\begin{array}{l}
\text{Gegeben:}\\\text{Winkel} \qquad \alpha \qquad [^{\circ}] \\
\text{Länge der Seite} \qquad c \qquad [m] \\
\text{Länge der Seite} \qquad b \qquad [m] \\
\\ \text{Gesucht:} \\\text{Länge der Seite} \qquad a \qquad [m] \\
\\ a = \sqrt{b^{2} +c^{2} -2\cdot b\cdot c\cdot cos\alpha }\\ \textbf{Gegeben:} \\ \alpha=150^{\circ} \qquad c=12m \qquad b=33m \qquad \\ \\ \textbf{Rechnung:} \\
a = \sqrt{b^{2} +c^{2} -2\cdot b\cdot c\cdot cos\alpha } \\
\alpha=150^{\circ}\\
c=12m\\
b=33m\\
a = \sqrt{(33m)^{2} +(12m)^{2} -2\cdot 33m \cdot 12m\cdot cos150^{\circ} }\\\\a=43,8m
\\\\\\ \small \begin{array}{|l|} \hline alpha=\\ \hline 150 ° \\ \hline 9\cdot 10^{3} \text{'} \\ \hline 5,4\cdot 10^{5} \text{''} \\ \hline 166\frac{2}{3} gon \\ \hline 2,62 rad \\ \hline \end{array} \small \begin{array}{|l|} \hline c=\\ \hline 12 m \\ \hline 120 dm \\ \hline 1,2\cdot 10^{3} cm \\ \hline 1,2\cdot 10^{4} mm \\ \hline 1,2\cdot 10^{7} \mu m \\ \hline \end{array} \small \begin{array}{|l|} \hline b=\\ \hline 33 m \\ \hline 330 dm \\ \hline 3,3\cdot 10^{3} cm \\ \hline 3,3\cdot 10^{4} mm \\ \hline 3,3\cdot 10^{7} \mu m \\ \hline \end{array} \small \begin{array}{|l|} \hline a=\\ \hline 43,8 m \\ \hline 438 dm \\ \hline 4,38\cdot 10^{3} cm \\ \hline 4,38\cdot 10^{4} mm \\ \hline 4,38\cdot 10^{7} \mu m \\ \hline \end{array} \end{array}$