18) (0,8ab−1/3c)(0,8ab+1/3c);

\((0.8ab-\frac13c)(0.8ab+\frac13c)=(0.8ab)^2-(\frac13c)^2=0.64a^2b^2-\frac19c^2\).

Ответ: \(0.64a^2b^2-\frac19c^2\).