`A=x^2/(x^4+x^2+1)`
`A=1/(x^2+1+1/x^2)`
`text(ta có : )x^2+1/x^2>=2sqrt(x^2+1/x^2)=2`
`text(để )M_max<=>x^2+1+1/x^2 min`
`text(mà )x^2+1/x^2>=2`
`tox^2+1/x^2=2`
`<=>x^4-2x^²+1=0`
`<=>(x^2)^2-2*x^²*1+1^2=0`
`<=>(x^2-1)^2=0`
`<=>x^2-1=0`
`<=>x^2=1`
`<=>`\(\left[ \begin{array}{l}x=1\\x=-1\end{array} \right.\)
`toB_max=1/3text( khi )x=+-1`
