a=in2/2 =Ln2^(1/2)
b=in3/3=Ln3^(1/3)
c=in5/5 =Ln5^(1/5)
2^(1/2)与3^(1/3)比较
两边同时6次方
左边=2^3=8
右边=3^2=9
所以 2^(1/2)