Câu 26:
\(A=\left(2\sqrt{7}-2\sqrt{3}-\sqrt{7}\right)\sqrt{7}+2\sqrt{21}\\ =\left(\sqrt{7}-2\sqrt{3}\right)\sqrt{7}+2\sqrt{21}=7-2\sqrt{21}+2\sqrt{21}=7\\ B=\sqrt{\dfrac{3}{75}}=\sqrt{\dfrac{1}{25}}=\dfrac{1}{5}\)
Câu 27:
\(a,\Leftrightarrow\sqrt{x}=4\Leftrightarrow x=16\\ b,\Leftrightarrow\sqrt{\left(x-2\right)^2}=3\Leftrightarrow\left|x-2\right|=3\\ \Leftrightarrow\left[{}\begin{matrix}x=3+2=5\\x=-3+2=-1\end{matrix}\right.\)
Câu 28:
\(a,\left\{{}\begin{matrix}x=\dfrac{6^2}{3}=12\\y=\sqrt{x\left(x+3\right)}=\sqrt{12\cdot15}=10\sqrt{3}\end{matrix}\right.\\ b,\left\{{}\begin{matrix}BC=\dfrac{AC}{\sin B}=\dfrac{5}{\sin50}\approx6,5\left(cm\right)\\AB=\sqrt{BC^2-AC^2}\approx4,2\left(cm\right)\end{matrix}\right.\)
