\[\begin {array} {1 1} (a)\;v_o \sqrt {\frac{m_1m_2}{v(m_1+m_2)}} & \quad (b)\;v_o \sqrt {\frac{2m_1m_2}{(m_1+m_2)R}} \\ (c)\;v_o \sqrt {\frac{m_1m_2}{2(m_1+m_2)k}} & \quad (d)\;v_o \sqrt {\frac{m_1m_2}{(m_1+m_2)k}} \end {array}\]