` *** 10. `

`1) (x+y)(x-3y) `

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

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

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

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

`= 6x^2 + xy - 2y^2 `

`3) (x+5)(5-x) `

`= 5x - x^2 + 25 - 5x `

`= -x^2 + 25 `

`4) (5x-2)(7x - 3) `

`= 35x^2 - 15x - 14x + 6 `

`= 35x^2 - 29x + 6 `

`5) (3+x)(x^2 - 3x + 9) `

`= 3x^2 - 9x + 27 + x^3 - 3x^2 + 9x `

`= x^3 + 27 `

`6) (2x +y)(4x^2 - 2xy + y^2) `

`= 8x^3 - 4x^2y + 2xy^2 + 4x^2y - 2xy^2 + y^3 `

`= 8x^3 + y^3 `