Đáp án:
$a)-\sqrt{x}y\sqrt{y}\\ b)-x\sqrt{\dfrac{-3x}{35}}\\ c)\dfrac{a\sqrt{5a}}{7\sqrt{b}}\\ d)-7\sqrt{3}\sqrt{xy}$
Giải thích các bước giải:
$a)-xy\sqrt{\dfrac{y}{x}} (x>0; y \ge 0)\\ =-xy\dfrac{\sqrt{y}}{\sqrt{x}} \\ =-\sqrt{x}y\sqrt{y}\\ b)\sqrt{\dfrac{-3x^3}{35}}(x<0)\\ =|x|\sqrt{\dfrac{-3x}{35}}\\ =-x\sqrt{\dfrac{-3x}{35}}\\ c)\sqrt{\dfrac{5a^3}{49b}}(a \ge 0; b >0)\\ =\dfrac{\sqrt{5a^3}}{\sqrt{49b}}\\ =\dfrac{|a|\sqrt{5a}}{7\sqrt{b}}\\ =\dfrac{a\sqrt{5a}}{7\sqrt{b}}\\ d)-7xy\sqrt{\dfrac{3}{xy}}(x<0;y<0)\\ =-7xy\dfrac{\sqrt{3}}{\sqrt{xy}}\\ =-7\sqrt{3}\sqrt{xy}$
