\frac{m_1}{\sqrt{\displaystyle 1-\frac{v_1^2}{c^2}}}+\frac{m_2}{\sqrt{\displaystyle 1-\frac{v_2^2}{c^2}}}=\frac{m_1}{\sqrt{\displaystyle 1-\frac{v_1'^2}{c^2}}}+\frac{m_2}{\sqrt{\displaystyle 1-\frac{v_2'^2}{c^2}}}.