a,

`(x^2-2x+1).(x-1)=x^2 .x-x^2-2x.x+2x.1+x-1=x^3-x^2-2x^2+2x+x-1=x^3-3x^2+3x-1`

c, 

`(x^3-2x^2+x-1).(5-x)`

`=5.x^3-2x^2 .5+5.x-5-x.x^3+2x^2 .x-x.x+x`

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

`=-x^4+7x^3-11x^2+6x-5`

e,

`(2x^3-3x-1).(5x+2)`

`=2x^3 .5x+2x^3 .2-3x.5x-3x.2-5x-2`

`=10x^4+4x^3-15x^2-6x-5x-2`

`=10x^4+4x^3-15x^2-11x-2`

`***` `***` `***`

a,

`(x^2-1).(x^2+2x)`

`=x^2 .x^2+x^2 .2x-x^2-2x`

`=x^4+2x^3-x^2-2x`

c,

`(x+3).(x^2+3x-5)`

`=x.x^2+x.3x-x.5+3.x^2+9x-15`

`=x^3+3x^2-5x+3x^2+9x-15`

`=x^3+6x^2+4x-15`

d,

`(x+1)(x^2-x+1)`

`=x.x^2-x.x+x+x^2-x+1`

`=x^3-x^2+x+x^2-x+1`

`=x^3+1`

__________________________

đa thức nhân đa thức:

`(a+b).(c+d)=a.c+a.d+b.c+b.d`