x''_2 \left( t \right) \cdot \cos \left( {x_1 \left( t \right) - x_2 \left( t \right)} \right) - x'_1 \left( t \right)\sin \left( {x_1 \left( t \right) - x_2 \left( t \right)} \right)\left( {x'_1 \left( t \right) - x'_2 \left( t \right)} \right)