Question

Xác định dấu của biểu thức :

\(A=\left(\frac{1}{6}\right)^{\log_62-\frac{1}{2}\log_{\sqrt{6}}5}-\sqrt[3]{\frac{31}{2}}\)

Answer 1

Ta có :

\(\log_62-\frac{1}{2}\log_{\sqrt{6}}5=\log_62-\log_65=\log_6\frac{2}{5}\)

\(\Rightarrow\left(\frac{1}{6}\right)^{\log_62-\frac{1}{2}\log_{\sqrt{6}}5}=\left(\frac{1}{6}\right)^{\log_6\frac{2}{5}}=\left(6^{-1}\right)^{\log_6\frac{2}{5}}=6^{\log_6\frac{2}{5}}=\frac{5}{2}=\sqrt[3]{\left(\frac{5}{2}\right)^3}=\sqrt[3]{\frac{125}{8}}\)

Mà :

\(\sqrt[3]{\frac{125}{8}}>\sqrt[3]{\frac{124}{8}}\Rightarrow\left(\frac{1}{6}\right)^{\log_62-\frac{1}{2}\log_{\sqrt{6}}5}>\sqrt[3]{\frac{31}{2}}\)

\(\Rightarrow B=\left(\frac{1}{6}\right)^{\log_62-\frac{1}{2}\log_{\sqrt{6}}5}-\sqrt[3]{\frac{31}{2}}>0^{ }\)