\(\left(x+\dfrac{1}{4}\right)\left(x+\dfrac{-1}{4}\right)\left(16x-1\right)\)
\(=\left(x+\dfrac{1}{4}\right)\left(x-\dfrac{1}{4}\right)\left(16x-1\right)\)
\(=\left[x^2-\left(\dfrac{1}{4}\right)^2\right]\cdot\left(16x-1\right)\)
\(=\left(x^2-\dfrac{1}{16}\right)\left(16x-1\right)\)
\(=x^2\left(16x-1\right)-\dfrac{1}{16}\left(16x-1\right)\)
\(=16x^3-x^2-x+\dfrac{1}{16}\)