`#AkaShi`
a) Với đk `a >= 0` thì:
`\sqrt{9a}-\sqrt{16a}+\sqrt{49a}`
`=3\sqrt{a}-4\sqrt{a}+7\sqrt{a}`
`=6\sqrt{a}`
.............
c) `(4\sqrt{x}-\sqrt{2x})(\sqrt{x}-\sqrt{2x})`
Với đk `x > 0`
`=4x-4x\sqrt{2}-x\sqrt{2}+2x`
`=6x-5x\sqrt{2}`
.......................
`d) (2\sqrt{x}+\sqrt{y})(3\sqrt{x}-2\sqrt{y})`
Với đk `x >=0 ; y >= 0`
`=6x-4\sqrt{xy}+3\sqrt{xy}-2y`
`=6x-\sqrt{xy}-2y`
...................
`2\sqrt{40\sqrt{12}}-2\sqrt{\sqrt{75}}-3\sqrt{5\sqrt{48}}`
`=2\sqrt{40.2\sqrt{3}}-2\sqrt{5\sqrt{3}}-3\sqrt{5.4\sqrt{3}}`
`=2\sqrt{16.5\sqrt{3}}-2\sqrt{5\sqrt{3}}-3\sqrt{5.4\sqrt{3}}`
`=8\sqrt{5\sqrt{3}}-2\sqrt{5\sqrt{3}}-6\sqrt{5\sqrt{3}}`
`=0`
