a. 5x ²(4x ²-2x+1) b. (3x ³-2)(-4x ²-x+3) c. (x^4-3x ³-6x ²+21x)-1

`a)5x^2(4x^2-2x+1)`

`=5x^2.4x^2+5x^2.(-2x)+5x^2.1`

`=20x^4-10x^3+5x^2.`

`b)(3x^3-2)(-4x^2-x+3)`

`=3x^3.(-4x^2)+3x^3.(-x)+3x^3.3+(-2).(-4x^2)+(-2).(-x)+(-2).3`

`=-12x^5-3x^4+9x^3+8x^2+2x-6`

`c)`Nếu là `-1:`

(x^4-3x^3-6x^2+21x)-1`

`=x^4-3x^3-6x^2+21x-1`

Nếu là `.(-1):`

`(x^4-3x^3-6x^2+21x).-1`

`=x^4+3x^3+6x^2-21x.`