Đáp án:
$a)A=0\\ b)B=-2y\\ c)C=1\\ d)D=2$
Giải thích các bước giải:
$a)A=\sqrt{\dfrac{-2t}{3}}.\sqrt{\dfrac{3t}{8}}( t\le 0)\\ \text{ĐKXĐ}: \left\{\begin{array}{l} -2t \ge 0\\ 3t \ge 0\end{array} \right.\\ \Leftrightarrow \left\{\begin{array}{l} t \le 0\\ t \ge 0\end{array} \right.\\ \Leftrightarrow t=0\\ \Rightarrow A=0\\ b)B=\dfrac{\sqrt{28y^6}}{\sqrt{7y^4}}(y<0)\\ =\sqrt{\dfrac{28y^6}{7y^4}}\\ =\sqrt{4y^2}\\ =2|y|\\ =-2y\\ c)C=\sqrt{x-\sqrt{x^2-1}}.\sqrt{x+\sqrt{x^2-1}}(x \ge 1)\\ =\sqrt{(x-\sqrt{x^2-1})(x+\sqrt{x^2-1})}\\ =\sqrt{x^2-(x^2-1)}\\ =1\\ d)\sqrt{\sqrt{x^4+4}-x^2}.\sqrt{\sqrt{x^4+4}+x^2}\\ =\sqrt{(\sqrt{x^4+4}-x^2)(\sqrt{x^4+4}+x^2)}\\ =\sqrt{x^4+4-x^4}\\ =\sqrt{4}\\ =2$
