\frac{2x^3+3 x}{x^4+x^2+1} = \frac{2x^3+ x}{x^4+x^2+1} + \frac{2x}{x^4+x^2+1} = \frac{2x^3+ x}{x^4+x^2+1} + \frac{2x}{(x^2+\frac{1}{2})^2 + \frac{3}{4} }