x^2/(x^4+x^2+1)=1/4
x^4+x^2+1=4x^2
x^4+1=3x^2
所以(5x^4-3x^2+5)/3x^2
=[5(x^4+1)-3x^2]/3x^2
=(5*3x^2-3x^2)/3x^2
=12x^2/3x^2
=4