`5A`

`a)`

`(x+1)(x^2-x+1)-(x+3)(x^2-3x+9)`

`=x^3+1^3-(x^3+3^3)`

`=x^3+1-(x^3+27)`

`=x^3+1-x^3-27`

`=-26`

`b)`

`(y+2)(y^2-2y+4)+(5-y)(25y+5y+y^2)`

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

`=y^3+8+125-y^3`

`=133`

`5B`

`a)`

`(x+2)(x^2-2x+4)-(1-3x)(1+3x+9x^2)`

`=x^3+2^3-[1^3-(3x)^3]`

`=x^3+8-(1-27x^3)`

`=x^3+8-1+27x^3`

`=28x^3+7`

`b)`

`(x+y)(x^2-xy+y^2)+(2y-x)(x^2+2xy+4y^2)`

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

`=x^3+y^3+8y^3-x^3`

`=9y^3`