f(x+y)-f(x)
=4y^2+8xy-y/(x^2+xy)
=4y^2+(y(8x^3-1)+8x^2*y^2)/(x^2+xy)
x>=0.58x^3-1>=0 y>0
f(x+y)-f(x)>0
函数f(x)=4x^2+1/x在[0.5,+∞)上为增函数