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

`=5x^2-3x^2-6x`

`=2x^2-6x`

`3, x^2-x(x+3)`

`=x^2-x^2-3x`

`=-3x`

`5, 3x(2x+1)-20=6x(x-2)+2`

`6x^2 +3x -20-6x^2+12x -2=0`

`15x-22=0`

`15x=22`

`x=22/15`

Vậy `x=22/15`

`7, x^3-5x^2+2x-x^2 +3x-5`

`=x^3-6x^2 +5x -5`

`9, x^4 +3x^3 +2x-x^2 -x+1`

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

`11, 5(3x^2 -4x) +9(2x^2-x) -2(x^2-5x)`

`=15x^2 -20x +18x^2 -9x -2x^2+10x`

`=31x^2 -19x`

`13, x(2x+1) -x^2(x+2)+(x^3 -x+3)`

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

`=3`