a) \(\dfrac{2}{\sqrt{7}-5}-\dfrac{2}{\sqrt{7}+5}=\dfrac{2\left(\sqrt{7}+5\right)-2\left(\sqrt{7}-5\right)}{\left(\sqrt{7}-5\right)\left(\sqrt{7}+5\right)}\)
\(=\dfrac{20}{\left(\sqrt{7}\right)^2-5^2}=-\dfrac{10}{9}\) là số hữu tỷ
b) \(\dfrac{\sqrt{7}+\sqrt{5}}{\sqrt{7}-\sqrt{5}}+\dfrac{\sqrt{7}-\sqrt{5}}{\sqrt{7}+\sqrt{5}}=\dfrac{\left(\sqrt{7}+\sqrt{5}\right)^2+\left(\sqrt{7}-\sqrt{5}\right)^2}{\left(\sqrt{7}-\sqrt{5}\right)\left(\sqrt{7}+\sqrt{5}\right)}\)
\(=\dfrac{7+2\sqrt{7.5}+5+7-2\sqrt{7.5}+5}{\left(\sqrt{7}\right)^2-\left(\sqrt{5}\right)^2}=\dfrac{24}{2}=12\) là số hữu tỷ
Đúng 1
Bình luận (0)
