`a)`

`A=(x-y)(x^2-xy)-x(x^2+2y^2)`

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

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

`A=-xy^2-2x^2y`

Thay `x=2;y=-3`, ta có:

`A=-2·(-3)^2-2·2^2·(-3)`

`A=-2·9-2·4·(-3)`

`A=-18+24`

`A=6`

`b)`

`B=(5x-2y)(25x^2+10xy+4y^2)`

`B=(5x-2y)[(5x)^2+2·5x·2y+(2y)^2]`

`B=(5x)^3-(2y)^3`

`B=125x^3-8y^3`

Thay `x=1/5;y=-1/2`, ta có:

`B=125·(1/5)^3-8·(-1/2)^3`

`B=125·1/125-8·(-1/8)`

`B=1+1`

`B=2`