1)rút gọn a)2√18 - 3√98 + 4√8 b)3√a - 2√25a +√36a với a≥0 c)5/4√8a^2/25 với a<0

Đáp án:

$a) 2\sqrt{18}$ - $3\sqrt{98}$ + $4\sqrt{8}$

= $2\sqrt{9.2}$ - $3\sqrt{49.2}$ + $4\sqrt{9.2}$

= $2.3\sqrt{2}$ - $3.7\sqrt{2}$ + $4.3\sqrt{2}$

= $6\sqrt{2}$ - $21\sqrt{2}$ + $12\sqrt{2}$

= $-3\sqrt{2}$

$b) 3\sqrt{a}$ - $2\sqrt{25a}$ + $\sqrt{36}$ (a$\geq$ 0)

= $3\sqrt{a}$ - $2.5\sqrt{a}$ + $6\sqrt{a}$

= $3\sqrt{a}$ - $10\sqrt{a}$ + $6\sqrt{a}$

= $-\sqrt{a}$

$\text{c) }$ $\dfrac{5}{4}$ . $\dfrac{\sqrt{8a}}{25}^{2}$ (a < 0)

= $\dfrac{1}{4}$ . $\dfrac{2\sqrt{2}a}{5}$

= $\dfrac{-\sqrt{2}.a}{10}$

#Ling