$\displaystyle \begin{array}{{>{\displaystyle}l}} a) \ 6\sqrt{12} -5\sqrt{27} +2\sqrt{48}\\ =6.2\sqrt{3} -5.3\sqrt{3} +2.4\sqrt{3}\\ =12\sqrt{3} -15\sqrt{3} +8\sqrt{3}\\ =5\sqrt{3}\\ b) \ \sqrt{\left( 1-2\sqrt{3}\right)^{2}} -\sqrt{4+2\sqrt{3}}\\ =|1-2\sqrt{3} |-\sqrt{3+2\sqrt{3} +1}\\ =2\sqrt{3} -1-\sqrt{\left(\sqrt{3} +1\right)^{2}}\\ =2\sqrt{3} -1-\sqrt{3} -1\\ =\sqrt{3} -2\\ Câu\ 2.\ \\ a) \ \sqrt{2x-15} =3\\ DK\ :x\geqslant \frac{15}{2} \ \\ \rightarrow 2x-15=9\\ \rightarrow 2x=24\\ \rightarrow x=12( tm) \ \\ b)\sqrt{x^{2} -2x+1} =5\\ \rightarrow \sqrt{( x-1)^{2}} =5\\ \rightarrow |x-1|=5\\ \rightarrow \left[ \begin{array}{l l} x-1=5 & \\ x-1=-5 & \end{array} \right.\rightarrow \left[ \begin{array}{l l} x=6 & \\ x=-4 & \end{array} \right. \end{array}$
