Rút gọn : $\sqrt{33-20\sqrt{2}}$

`\sqrt{33 - 20 \sqrt{2} }`

` = \sqrt{25 - 20 \sqrt{2} + 8}`

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

` = \sqrt{(5 - 2 \sqrt{2})^2}`

` = |5 - 2 \sqrt{2}|`

` = 5 - 2 \sqrt{2}`