b) `x/4-\sqrt{x}\sqrt{y}+y`
`=((\sqrt{x})/(2))^2-2.(\sqrt{x})/(2).\sqrt{y}+(\sqrt{y})^2`
`=((\sqrt{x})/(2)-\sqrt{y})^2`
c) `8x\sqrt{x}-27y\sqrt{y}`
`=(\sqrt{4x})^3-(\sqrt{9y})^3`
`=(\sqrt{4x}-\sqrt{9y})[(\sqrt{4x})^2+\sqrt{4x}.\sqrt{9y}+(\sqrt{9y})^2]`
`=(\sqrt{4x}-\sqrt{9y})(4x+6\sqrt{x}\sqrt{y}+9y)`
`=(2\sqrt{x}-3\sqrt{y})(4x+6\sqrt{x}\sqrt{y}+9y)`
d) `\sqrt{x^2-9}-3\sqrt{x-3}`
`=\sqrt{(x-3)(x+3)}-3\sqrt{x-3}`
`=\sqrt{x-3}(\sqrt{x+3}-3)`
