Rút gọn biểu thức sau: (a) 5x^2y^4.(−1/20xy^3)

a) \[5{x^2}{y^4}.\left( {\frac{{ - 1}}{{20}}x{y^3}} \right)\]\[ = \frac{{ - 1}}{4}{x^3}{y^7}\]

b) \[\left( {{x^3}y + 4{x^2}{y^2} - 5{x^2}{y^3}} \right):xy - x\left( {x - 4{y^2}} \right)\].

\[ = {x^2} + 4xy - 5x{y^2} - {x^2} + 4x{y^2}\]

\[ = 4xy - x{y^2}\]

c) \[{\left( {x + 1} \right)^2} - \left( {2x - 1} \right)\left( {x + 3} \right) + \left( {x - 5} \right)\left( {x + 5} \right)\].

\[ = {x^2} + 2x + 1 - 2{x^2} - 5x + 3 + {x^2} - 25\]

\[ = - 3x - 21\]