Verified answer

1) 5m(m - n)(m + 3n) = (5m² - 5mn)(m + 3n) = 5m³ + 15m²n - 5m²n - 15mn² =

= 5m³ + 10m²n - 15mn²

2) (m² - m - 1)(m² + m + 1) = m⁴ + m³ + m² - m³ - m² - m - m² - m - 1 =

= m⁴ - m² - 2m - 1

второй способ :

(m² - m - 1)(m² + m + 1) = [m² - (m + 1)][m² + (m + 1)] = (m²)² - (m + 1)² =

= m⁴ - m² - 2m - 1