$\text{Rút gọn các biểu thức sau:}$
$\text{a)}$ $A= (4x-1)(3x+1)-5x(x-3)-(x-4)(x-3)$
$A = (12x² + 4x - 3x - 1) -5x² + 15x - (x² - 3x - 4x + 12)$
$A = 12x² + x - 1 - 5x² + 15x - x² + 7x - 12$
$A = 6x² + 23x - 13$
$\text{b)}$ $B = (5x - 2).(x + 1) -3x.(x² - x - 3) -2x.(x - 5).(x - 4)$
$B = 5x² + 5x - 2x - 2 - 3x³ + 9x + (-2x² + 10x)×(x - 4)$
$B = 5x² + 5x - 2x - 2 - 3x³ + 9x - 2x³ + 8x² + 10x² - 40x$
$B = -5x³ + 26x² - 28x - 2$