E_k=mc^2-m_0c^2=m_0c^2\left(\frac 1{\sqrt{1-v^2/c^2}}-1\right)=\frac{1-\sqrt{1-v^2/c^2}}{\sqrt{1-v^2/c^2}}m_0c^2=\frac{m_0v^2}{\sqrt{1-v^2/c^2}(1+\sqrt{1-v^2/c^2}))}