Câu 6:

`a)(x^2y-2xy)*(-3x^2y)`

`=x^2y*(-3x^2y)-2xy*(-3x^2y)`

`=-3*(x^2*x^2)*(y*y)+(2*3)*(x*x^2)*(y*y)`

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

`b)x^2(x-y)+y(x^2+y)`

`=(x^2*x-x^2*y)+(y*x^2+y*y)`

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

`=x^3+y^2`

`c)x(4x^2-5xy+2x)`

`=x*4x^2-x*5xy+x*2x`

`=4(x*x^2)-5*(x*x)*y+2(x*x)`

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

`d)x^2(x+y)+2x(x^2+y)`

`=(x^2*x+x^2*y)+(2x*x^2+2x*y)`

`=x^3+x^2y+2x^3+2xy`

`=(x^3+2x^3)+x^2y+2xy`

`=3x^3+x^2y+2xy`