TeX source:
\begin{array}{rcl} f'_{cb}(a,b,c) &=& 25c\cdot\left(-\dfrac{1}{2}\right)\left(16a^2-b^2+25c^2\right)^{-\frac{1}{2}-1}\cdot (-2b) \\\\&=& 25bc\left(16a^2-b^2+25c^2\right)^{-\frac{3}{2}-1} \\\\&=& \dfrac{25bc}{\left(16a^2-b^2+25c^2\right)^{\frac{3}{2}}} \\\\&=& \dfrac{25bc}{\left(\sqrt{16a^2-b^2+25c^2}\right)^3}\end{array}