2) Ta có: \(a=\left(-\dfrac{1}{5}\right)^{84}\)
\(=\left[\left(-\dfrac{1}{5}\right)^2\right]^{42}\)
\(=\left(\dfrac{1}{25}\right)^{42}\)
\(=\left(\dfrac{1}{625}\right)^{21}\)
Ta có: \(b=\left(-\dfrac{1}{3}\right)^{126}\)
\(=\left[\left(-\dfrac{1}{3}\right)^3\right]^{42}\)
\(=\left(-\dfrac{1}{27}\right)^{42}\)
\(=\left(\dfrac{1}{729}\right)^{21}\)
Vì \(\dfrac{1}{625}>\dfrac{1}{729}\) nên \(\left(\dfrac{1}{625}\right)^{21}>\left(\dfrac{1}{729}\right)^{21}\)
hay a>b
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