Thực hiện các phép tính: (a) 4x(x^2−5y). (b) (12x^2y−8xy^2+xy):(4xy).

\[\begin{array}{l}a.\,\,\,\,4x\left( {{x^2} - 5y} \right)\\ = 4{x^3} - 20xy\end{array}\]

\[\begin{array}{l}b.\,\,\,\,\left( {12{x^2}y - 8x{y^2} + xy} \right):\left( {4xy} \right)\\ = 3x - 2y + \frac{1}{4}\end{array}\]

\[\begin{array}{l}c.\,\,\,\,{\left( {x - 3} \right)^2} - \left( {25x + 4} \right).\left( { - 13 + 5x} \right)\\ = {x^2} - 6x + 9 - \left( { - 325x + 125{x^2} - 52 + 20x} \right)\\ = {x^2} - 6x + 9 + 325x - 125{x^2} + 52 - 20x\\ = - 124{x^2} + 299x + 61\end{array}\]