1) m³ - n³ + 2n - 2m = ( m³ - n³) + 2( n -m) = ( m - n)(m² +mn + n²) - 2( m - n) =
= ( m - n)(m² + mn + n² - 2)
2) x⁶ + y⁶ + x² + y² = (x²)³ + (y²)³ + ( x² + y²) = ( x² + y²)(x⁴ - x²y² + y⁴) + ( x² + y²) =
= (x² + y²)( x⁴ - x²y² + y⁴ + 1)
3) x⁴ + xy3 - x3y - y⁴ = x⁴ + 3xy - 3xy - y⁴ = x⁴ - y⁴ = ( x²)² - (y²)² = (x² - y²)(x² + y²).