1. (xy + x²y²)(x²y + xy²) = x³y² + x²y³ + x⁴y³ + x³y⁴
2. (2a² - 5b²)(3a² + 4b²) = 6a⁴ + 8a²b² - 15a²b² - 20b⁴ = 6a⁴ - 7a²b² - 20b⁴
3. (5x² - 6y²)(6x² - 5y²) = 30x⁴ - 25x²y² - 36y⁴ + 30x²y² = 30x⁴ + 5x²y² - 36y⁴
4. (x² + 2x + 1)(x + 3) = x³ + 2x² + x + 3x² + 6x + 3 = x³ + 5x² + 7x + 3