a=(x^3-1)/x^2=x-1/x^2,
因a>=0,得x>=1,
x=1时,a=0
x=2时,a=2-1/4=7/4
x=3时,a=3-1/9=26/9
x=4时,a=4-1/16=63/16
x>=1增大时,x-1/x^2单调增
因为只有3个a值,故26/9=