18) \((0,8ab-\frac13c)(0,8ab+\frac13c)\)

\((0.8ab-\frac13c)(0.8ab+\frac13c)=(0.8ab)^2-(\frac13c)^2\).

\(=0.64a^2b^2-\frac19c^2\).

Ответ: \(0.64a^2b^2-\frac19c^2\).