18. \(\frac{4}{9}x² − 1\)

\(\frac{4}{9}x^2-1=\left(\frac{2x}{3}\right)^2-1^2=\left(\frac{2x}{3}-1\right)\left(\frac{2x}{3}+1\right)\).

Ответ: \(\left(\frac{2x}{3}-1\right)\left(\frac{2x}{3}+1\right)\).