1.

\left. \begin{array} { c } { ( b + c - a ) \operatorname { tan } \frac { A } { 2 } = ( c + a - b ) \operatorname { tan } \frac { B } { 2 } } \\ { = ( a + b - c ) \operatorname { tan } \frac { C } { 2 } } \end{array} \right.

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