a, 128 = 122.4 = (122)4 = 1444
812 = 83.4 = (83)4 = 5124
Vì 5124 > 1444
=> 812 > 128
b, (-5)39 = (-5)3.13 = [(-5)3]13 = (-125)13 = -12513
(-2)91 = (-2)7.13 = [(-2)7]13 = (-128)13 = -12813
Có 12513 < 12813
=> -12513 > -12813
=> (-5)39 > (-2)91
Có
a) \(12^8=\left(3.2^2\right)^8=3.2^{16}\)
\(8^{12}=\left(2^3\right)^{12}=2^{36}\)
vì \(2^{36}>3.2^{16}\)
\(\Rightarrow12^8< 8^{12}\)
b)\(\left(-5\right)^{39}=\left(-5\right)^{30}.\left(-5\right)^9=\left(-5^3\right)^{10}.\left(-5\right)^9=\left(-125\right)^{10}.\left(-5\right)^9\)
\(\left(-2\right)^{91}=\left(-2\right)^{90}.2=\left(-2^9\right)^{10}.2=\left(-512\right)^{10}.2\)
vì \(-125>-512\Rightarrow-125^{10}>-512^{10}\)
vì \(\left(-5\right)^9>2\)
\(\Rightarrow\left(-5\right)^{39}>\left(-2\right)^{91}\)
