Thu gọn đa thức: a) 5x^4 – 2x^3y + 20xy^3 + 6x^3y – 3x^2y^2 + xy^3 – y^4;

a) 5x⁴ – 2x³y + 20xy³ + 6x³y – 3x²y² + xy³ – y⁴

= 5x⁴ + (6x³y – 2x³y) + (20xy³ + xy³) – 3x²y² – y⁴

= 5x⁴ + 4x³y + 21xy³ – 3x²y² – y⁴.