Đáp án:
Giải thích các bước giải:
$\sqrt{125}-4\sqrt{45}+3\sqrt{20}-\sqrt{80}$
$=\sqrt{25.5}-4\sqrt{9.5}+3\sqrt{4.5}-\sqrt{16.5}$
$=5\sqrt{5}-12\sqrt{5}+6\sqrt{5}-4\sqrt{5}$
$=(5-12+6-4)\sqrt{5}$
$=-5\sqrt{5}$
$3.$
$2\sqrt{2x}-5\sqrt{8x}+7\sqrt{18x}=28$
$⇔2\sqrt{2x}-5\sqrt{4.2x}+7\sqrt{9.2x}=28$
$⇔2\sqrt{2x}-10\sqrt{2x}+21\sqrt{2x}=28$
$⇔13\sqrt{2x}=28$
$⇔\sqrt{2x}=\dfrac{28}{13}$
$⇔2x=\dfrac{784}{169}$
$⇔x=\dfrac{784}{338}=\dfrac{392}{169}$