Rút gọn các biểu thức sau: (a) (x−3)(5x+1)−5x(x+6). (b) (1+5x)^2−(3x−1)(3x+1).

a) \(\left( {x - 3} \right)\left( {5x + 1} \right) - 5x\left( {x + 6} \right)\)

\[ = 5{x^2} + x - 15x - 3 - 5{x^2} - 30x\]

\[ = - 44x - 3\]

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

\( = 1 + 10x + 25{x^2} - 9{x^2} + 1\)

\( = 16{x^2} + 10x + 2\)