`a) x^2 - 2\sqrt{7}x + 7`
`= x^2 - 2\sqrt{7} + \sqrt{7}^2`
`=(x -\sqrt{7})^2`
`b) x -4\sqrt{x} + 4 ( x >0)`
`= x -2.2\sqrt{x} + 2^2`
`=(x -2)^2`
`c) x\sqrt{x} +y \sqrt{y}`
`= \sqrt{x}^3 + \sqrt{y}^3`
`= (\sqrt[x} + \sqrt{y}) ( x - \sqrt{xy} + y)`
`d) a -2 \sqrt{a} (a>0)`
`= \sqrt{a}^2 - 2\sqrt{a}`
`= \sqrt{a}(\sqrt{a} -2)`
`e) x -3( x>0)`
`= \sqrt{x}^2 - \sqrt{3}^2 `
`= ( \sqrt{x} -\sqrt{3})(\sqrt{x} + \sqrt{3})`
#hoctot
