Đáp án+Giải thích các bước giải:

 `a)(2-\sqrt{2}).(-5\sqrt{2})-(3\sqrt{2}-5)^{2}`

`=-10\sqrt{2}+5.\sqrt{2^{2}}-[(3\sqrt{2})^{2}-2.3\sqrt{2}.5+5^{2}]`

`=-10\sqrt{2}+5.2-(9.2-30\sqrt{2}+25)`

`=-10\sqrt{2}+10-18+30\sqrt{2}-25`

`=20\sqrt{2}-33`

`b)2\sqrt{3a}-\sqrt{75a}+a\sqrt{\frac{13,5}{2a}}-2/5 . \sqrt{300a^{3}}` `(đk : a>0)`

`=2\sqrt{3a}-\sqrt{5^{2}.3a}+\sqrt{\frac{13,5a^{2}}{2a}}-2/5. \sqrt{(10a)^{2}.3a}`

`=2\sqrt{3a}-5\sqrt{3a}+\sqrt{\frac{13,5}{2}a}-2/5. 10a. \sqrt{3a}`

`=-3\sqrt{3a}+\sqrt{27/4a}-4a\sqrt{3a}`

`=\sqrt{3a}.(-3-4a)+\sqrt{3a. (3/2)^{2}}`

`=\sqrt{3a}.(-3-4a)+3/2. \sqrt{3a}`

`=\sqrt{3a}.(-3-4a+3/2)`

`=\sqrt{3a}.(-3/2-4a)`