`b)\sqrt{50x^6y^3}:\sqrt{2x^2y}`

`= \sqrt{(50x^6y^3)/(2x^2y)}`

`= \sqrt{25x^4y^2} `

`= \sqrt{25} × \sqrt{x^4} × \sqrt{y^2} `

`= 5 × x^2 × y `

`= 5x^2y`

`c) \sqrt{2xy^3} . \sqrt{2xy} `

`= \sqrt{(2xy^3)(2xy)} `

`= \sqrt{4x^2y^4}`

`= \sqrt{4} × \sqrt{x^2} × \sqrt{y^4} `

`= 2 × x × y^2 `

`= 2xy^2 `

`d) \sqrt{5x^3y} × \sqrt{125xy}`

`= \sqrt{(5x^3y)(125xy)} `

`= \sqrt{625x^4y^2} `

`= \sqrt{625} × \sqrt{x^4} × \sqrt{y^2} `

`= 25 × |x^2| × |y| ` (Vì `x < 0, y < 0 => x^2 > 0, |x^2| = x^2; |y| = -y`)

`=-25x^2y`