3^0.8和3^0.7
对两个数取ln,所以3^0.8大于3^0.7.
0.75^-1和0.75^0.1
同样的方法-ln0.75大于0.1ln0.75,所以0.75^-1大于0.75^0.1.
0.9^-8小于0.8^-9