390. 1) (2x² - 5xy + y²) 2xy; 2) (-6a² - 7ab - b²) • (-3ab); 3) (5m² + 10mn - 4n²) • \(-\frac{1}{2}mn\); 4) \(-\frac{1}{3}n² - pq + q²\) • (-2pq).

Решение:

1. \( (2x^2 - 5xy + y^2) \cdot 2xy = 4x^3y - 10x^2y^2 + 2xy^3 \)
2. \( (-6a^2 - 7ab - b^2) \cdot (-3ab) = 18a^3b + 21a^2b^2 + 3ab^3 \)
3. \( (5m^2 + 10mn - 4n^2) \cdot (-\frac{1}{2}mn) = -\frac{5}{2}m^3n - 5m^2n^2 + 2mn^3 \)
4. \( (-\frac{1}{3}n^2 - pq + q^2) \cdot (-2pq) = \frac{2}{3}n^2pq + 2p^2q^2 - 2pq^3 \)

Ответ: 1) 4x³y - 10x²y² + 2xy³; 2) 18a³b + 21a²b² + 3ab³; 3) -\(\frac{5}{2}\)m³n - 5m²n² + 2mn³; 4) \(\frac{2}{3}\)n²pq + 2p²q² - 2pq³.