\(v = (1 - 4 \,cos\,5y)^{\frac{-1}{2}}\)
\(\frac{dv}{dy} = \frac{-1}{2}(1 - 4\,cos\,5y)^{\frac{-3}2} \,\frac d{dy}(1 - 4\,cos \,5y)\)
\(= \frac{-1}{2}(1 -4\, cos \,5y)^{\frac{-3}2} \,x -4x - 5 sin\,5y\)
\(= - 10 \,sin\,5y(1 -4 \,cos\,5y)^{\frac{-3}{2}}\)