Đáp án+Giải thích các bước giải:

 `a)x.(x+1)=x.x+x.1=x^{2}+x`

`b)2x.(6-3x)`

`=2x.6-2x.3x`

`=12x-6x^{2}`

`c)(x+2).(x-4)`

`=x.(x-4)+2.(x-4)`

`=x^{2}-4x+2x-8`

`=x^{2}-2x-8`

`d)(x-5).(7+x)`

`=x.(7+x)-5.(7+x)`

`=7x+x^{2}-35-5x`

`=x^{2}+2x-35`

`e)(2y+3).(4-y)`

`=2y.(4-y)+3.(4-y)`

`=8y-2y^{2}+12-3y`

`=-2y^{2}+5y+12`

`f)(5-3a).(4+a)`

`=5.(4+a)-3a.(4+a)`

`=20+5a-12a-3a^{2}`

`=-3a^{2}-7a+20`