1) $\sqrt{63}$ = 9$\sqrt{7}$ ( sai )
Sửa: $\sqrt{63}$ = 3$\sqrt{7}$
Vì 3$\sqrt{7}$ = $\sqrt{9.7}$ = $\sqrt{63}$
mà 9$\sqrt{7}$ = $\sqrt{81.7}$ = $\sqrt{567}$
2) $\sqrt{72}$ = 6$\sqrt{2}$ ( đúng )
3) -7$\sqrt{2}$ = $\sqrt{(-7)^{2}.2}$ = $\sqrt{28}$ (sai)
Sửa: -7$\sqrt{2}$ = $\sqrt{(-7)^{2}.2}$ = $\sqrt{98}$
Vì -7$\sqrt{2}$ = $\sqrt{(-7)^{2}.2}$ = $\sqrt{49.2}$ = $\sqrt{98}$
4) 2$\sqrt{\frac{3}{2}}$ = $\sqrt{3}$ ( sai )
Sửa: 2$\sqrt{\frac{3}{2}}$ = $\sqrt{6}$
Vì 2$\sqrt{\frac{3}{2}}$ = $\sqrt{4.\frac{3}{2}}$ = $\sqrt{6}$
5) $\sqrt{4.18.a}$ = 6$\sqrt{2a}$ (a$\geq$0) ( đúng )
Vì $\sqrt{72a}$ = $\sqrt{36.2a}$ = $\sqrt{6^{2}.2a}$ = 6$\sqrt{2a}$
