[tex]\it 4)\ a=\dfrac{^{\sqrt3)}1}{2\sqrt3}-4\sqrt{13^2-12^2}=\dfrac{\sqrt3}{2\cdot3}-4\sqrt{169-144}=\dfrac{\sqrt3}{6}-20\\ \\ \\ b=\dfrac{1}{2}(14-\dfrac{\sqrt3}{3})-(-3)^3=7-\dfrac{\sqrt3}{6}+27=34-\dfrac{\sqrt3}{6}\\ \\ \\ m_a=\dfrac{\dfrac{\sqrt3}{6}-20+34-\dfrac{\sqrt3}{6}}{2}=\dfrac{14}{2}=7[/tex]
