`2)(a-3)(5-a)`
`=5a-a²-15+3a`
`=-a²+(5a+3a)-15`
`=-a²+8a-15`
`4)(x-2y)(x+y)`
`=x²+xy-2xy-2y²`
`=x²-2y²+(xy-2xy)`
`=x²-2y²-xy`
`6)(x-3)(x²+x+1)`
`=x³+x²+x-3x²-3x-3`
`=x³+(x²-3x²)+(x-3x)-3`
`=x³-2x²-2x-3`
`8)(x-6)(x+y-z)`
`=x²+xy-xz-6x-6y+6z`
`10)-(x+2)(x²+2x+2)`
`=-(x³+2x²+2x+2x²+4x+4)`
`=-[x³+(2x²+2x²)+(2x+4x)+4]`
`=-(x³+4x²+6x+4)`
`=-x³-4x²-6x-4`
`12)x²-2x-(x+7)(x+2)`
`=x²-2x-(x²+2x+7x+14)`
`=x²-2x-x²-2x-7x-14`
`=(x²-x²)-(2x+2x+7x)-14`
`=-11x-14`
`14)(x-4)(x-1)+(x-2)(3-x)`
`=x²-x-4x+4+3x-x²-6+2x`
`=(x²-x²)+(-x-4x+3x+2x)+(4-6)`
`=-2`
`16)(x-2)(x+3)-(x+1)(2x-1)`
`=x²+3x-2x-6-(2x²-x+2x-1)`
`=x²+3x-2x-6-2x²+x-2x+1`
`=(x²-2x²)+(3x-2x+x-2x)+(-6+1)`
`=-x²-5`
`18)5x-3+(x-5)(x+4)`
`=5x-3+x²+4x-5x-20`
`=x²+(5x+4x-5x)-(3+20)`
`=x²+4x-23`
`20)2(-x-2)(x+3)+3x²`
`=(-2x-4)(x+3)+3x²`
`=-2x²-6x-4x-12+3x²`
`=(-2x²+3x²)-(6x+4x)-12`
`=x²-10x-12`
--------------------------------------
`2)3x-(x-3)(x+1)=-x²`
`⇔3x-(x²+x-3x-3)=-x²`
`⇔3x-x²-x+3x+3=-x²`
`⇔3x-x²-x+3x+x²=-3`
`⇔5x=-3`
`⇔x=-3/5`
Vậy `x=-3/5`
`4)(2x+1)(x-3)-2x(x+7)=100`
`⇔2x²-6x+x-3-2x²-14x=100`
`⇔2x²-6x+x-2x²-14x=100+3`
`⇔-19x=103`
`⇔x=-103/19`
Vậy `x=-103/19`
`6)(x-1)(x-2)=(x-3)(x+4)`
`⇔(x-1)(x-2)-(x-3)(x+4)=0`
`⇔x²-2x-x+2-(x²+4x-3x-12)=0`
`⇔x²-2x-x+2-x²-4x+3x+12=0`
`⇔-4x+14=0`
`⇔-4x=-14`
`⇔x=7/2`
Vậy `x=7/2`