a: \(=\left(4x^4\right)^3-\left(2x+2y\right)^3\)
\(=\left(4x^4-2x-2y\right)\left(16x^8+4x^4\left(2x+2y\right)+\left(2x+2y\right)^2\right)\)
b: \(=\left(\dfrac{1}{5}m^3\right)^3+\left(3m^5\right)^3\)
\(=\left(\dfrac{1}{5}m^3+3m^5\right)\left(\dfrac{1}{25}m^6+\dfrac{3}{5}m^8+9m^{10}\right)\)