TeX source:

\mathrm{T}^{\prime}(\mathrm{x})=\frac{1}{3} \frac{1}{2}\left(\mathrm{x}^{2}+16\right)^{-1 / 2}(2 \mathrm{x})-\frac{1}{5}=\frac{\mathrm{x}}{3 \sqrt{\mathrm{x}^{2}+16}}-\frac{1}{5}