Đáp án:
b) $\frac{-10}{9}$
c) $2\sqrt{5}$
Giải thích các bước giải:
b) $\frac{2}{\sqrt{7}-5}-\frac{2}{\sqrt{7}+5}=\frac{2(\sqrt{7}+5)}{(\sqrt{7}-5)(\sqrt{7}+5)}-\frac{2(\sqrt{7}-5)}{(\sqrt{7}+5)(\sqrt{7}-5)}\\
=\frac{2\sqrt{7}
+10-2\sqrt{7}+10}{7-25}=\frac{20}{-18}=\frac{-10}{9}$
c) $\sqrt{14+6\sqrt{5}}-\sqrt{14-6\sqrt{5}}=\sqrt{3^2+2.3\sqrt{5}+
\sqrt{5}^2}-\sqrt{3^2-2.3\sqrt{5}+
\sqrt{5}^2}\\
=\sqrt{(3+\sqrt{5})^2}-\sqrt{(3-\sqrt{5})^2}=3+\sqrt{5}-(3-\sqrt{5})=2\sqrt{5}$
