\(A=\left(\sqrt{7}-9\sqrt{7}+2\sqrt{7}\right)\sqrt{7}=-6\sqrt{7}\cdot\sqrt{7}=-42\\ B=\sqrt{\left(\sqrt{10}-2\right)^2}-2\sqrt{\left(\sqrt{10}+2\right)^2}\\ B=\sqrt{10}-2-2\sqrt{10}-4=-6-\sqrt{10}\\ C=3\sqrt{5}+2\sqrt{5}-10\sqrt{5}=-5\sqrt{5}\)
i. \(\Rightarrow A=\left(\sqrt{7}-9\sqrt{7}+2\sqrt{7}\right)\sqrt{7}\\ \Rightarrow A=-6\sqrt{7}.\sqrt{7}\\ \Rightarrow A=-42\)
ii. \(\Rightarrow B=\sqrt{14-2\sqrt{40}}-2\sqrt{14+2\sqrt{40}}\\ \Rightarrow\sqrt{\left(\sqrt{10}-2\right)^2}-2\sqrt{\left(\sqrt{10}+2\right)^2}\\ \Rightarrow B=\sqrt{10}-2-2\left(\sqrt{10}+2\right)\\ \Rightarrow B=\sqrt{10}-2-2\sqrt{10}-4\\ \Rightarrow B=-6-\sqrt{10}\)
iii. \(\Rightarrow C=3\sqrt{5}+2\sqrt{5}-10\sqrt{5}\\ \Rightarrow C=-5\sqrt{5}\)
Câu 2:
\(a,A=\left(\sqrt{7}-3\sqrt{63}+\sqrt{28}\right).\sqrt{7}\)
\(=\left(\sqrt{7}-9\sqrt{7}+2\sqrt{7}\right).\sqrt{7}\)
\(=-6\sqrt{7}.\sqrt{7}\)
\(=-42\)
\(b,B=\sqrt{14-4\sqrt{10}}-2\sqrt{14+4\sqrt{10}}\)
\(=\sqrt{\left(\sqrt{10}\right)^2-2.\sqrt{10}.2+2^2}-2\sqrt{\left(\sqrt{10}\right)^2+2.\sqrt{10}.2+2^2}\)
\(=\sqrt{\left(\sqrt{10}-2\right)^2}-2\sqrt{\left(\sqrt{10}+2\right)^2}\)
\(=\left|\sqrt{10}-2\right|-2\left|\sqrt{10}+2\right|\)
\(=\sqrt{10}-2-2\left(\sqrt{10}+2\right)\)
\(=\sqrt{10}-2-2\sqrt{10}-4\)
\(=-\sqrt{10}-6\)
\(c,C=3\sqrt{5}+\sqrt{20}-2\sqrt{125}\)
\(=3\sqrt{5}+2\sqrt{5}-10\sqrt{5}\)
\(=-5\sqrt{5}\)
