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| 1. |
\left. \begin{array} { l } { \operatorname { cos } A + \operatorname { cos } B - \operatorname { cos } C } \\ { = 4 \cdot \operatorname { cos } ( \frac { A } { 2 } ) \cdot \operatorname { cos } ( \frac { B } { 2 } ) \cdot \operatorname { sin } ( \frac { C } { 2 } ) - 1 } \end{array} \right. |
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