`#khanhly2949`

`a,`

`A=2\sqrt{18}+3\sqrt{72}-3\sqrt{32}`

`=6\sqrt{2}+18\sqrt{2}-12\sqrt{2}`

`=24\sqrt{2}-12\sqrt{2}`

`=12\sqrt{2}`

`b,`

`P=(1/(\sqrt{x}-5)-1/(sqrt{x}+5)) : 5/(x-10\sqrt{x}+25)`   ĐK `:x>=0 ; x\ne 25`

`=[(\sqrt{x}+5)/((\sqrt{x}-5)(\sqrt{x}+5))-(\sqrt{x}-5)/((\sqrt{x}-5)(\sqrt{x}+5))] : 5/(\sqrt{x}-5)^2`

`=(\sqrt{x}+5-\sqrt{x}+5)/((\sqrt{x}-5)(\sqrt{x}+5)) . (\sqrt{x}-5)^2/5`

`=10/((\sqrt{x}-5)(\sqrt{x}+5)) . (\sqrt{x}-5)^2/5`

`=(2(\sqrt{x}-5))/(\sqrt{x}+5)`

`=(2\sqrt{x}-10)/(\sqrt{x}+5)`