a, 3x.(4x-3)-(2x-1)-(x+3)
= 12x² -9x -2x+1 -x -3
= 12x² -12x -2
b, 4x.(3x ²-x)-(2x+3).(6x ²-3x+1)
= 12x³ -4x² - ( 12x³ -6x² +2x + 18x² -9x +3)
= 12x³ -4x² - 12x³ + 6x² - 2x - 18x² +9x -3
=- 16x² +7x -3
c, (x-2).(x+2).(x+4)
= (x-2). ( x² +4x +2x +8)
= (x-2).(x² +6x+8)
= x³ + 6x² +8x -2x² -12x -16
= x³ + 4x² -4x -16
d, [(x ²-2xy+2y ²).(x+2y)-( x ³+4y ³)].(-3xy)
= x³ +2x²y -2x²y -4xy² +2xy² +4y³ + 3xy.(x³ +4y³)
= x³+4y³ -2xy² + 3$x^{4}$y + 12x$y^{4}$
Chúc chủ tus học giỏi điểm cao nhé ^^