19. (2,7x³ + 2,8y³)(2,7x³ − 2,8y³)

\((2.7x^3+2.8y^3)(2.7x^3-2.8y^3)=(2.7x^3)^2-(2.8y^3)^2\).

\(=7.29x^6-7.84y^6\).

Ответ: \(7.29x^6-7.84y^6\).