a) $\sqrt{120}$ - ($\sqrt{5}$ + $\sqrt{6}$)²
= $\sqrt{30.4}$ - (5 + 2$\sqrt{30}$ + 6)
= 2$\sqrt{30}$ - 5 - 2$\sqrt{30}$ - 6
= -11
5$\sqrt{18}$ - $\sqrt{50}$ + $\sqrt{8}$
= 5$\sqrt{9.2}$ - $\sqrt{25.2}$ + $\sqrt{4.2}$
= 5.3.$\sqrt{2}$ - 5$\sqrt{2}$ + 2$\sqrt{2}$
= $\sqrt{2}$.(15 - 5 + 2)
= 12$\sqrt{2}$
b) $\sqrt{x + 1}$ - $\sqrt{4x + 4}$ + $\sqrt{9x + 9}$ = 1 Đkxđ: x ≥ -1
⇔ $\sqrt{x + 1}$ - $\sqrt{4.(x + 1)}$ + $\sqrt{9.(x + 1)}$ = 1
⇔ $\sqrt{x + 1}$ - 2$\sqrt{x + 1}$ + 3$\sqrt{x + 1}$ = 1
⇔ $\sqrt{x + 1}$.(1 - 2 + 3) = 1
⇔ 2$\sqrt{x + 1}$ = 1
⇔ $\sqrt{x + 1}$ = $\frac{1}{2}$
⇔ x + 1 = $\frac{1}{4}$
⇔ x = -$\frac{3}{4}$ (t/m)
Vậy x = -$\frac{3}{4}$
