`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.`