Cho đa thức P = 3x²y – 2xy² – 4xy + 2.

a) Tìm đa thức Q sao cho Q – P = –2x³y + 7x²y + 3xy.

a) Ta có: Q – P = –2x³y + 7x²y + 3xy.

Suy ra Q = P + (–2x³y + 7x²y + 3xy)

               = 3x²y – 2xy² – 4xy + 2 –2x³y + 7x²y + 3xy

               = (3x²y + 7x²y) – 2xy² + (– 4xy + 3xy) + 2 –2x³y

               = 10x²y – 2xy² – xy + 2 –2x³y.

Vậy Q = 10x²y – 2xy² – xy + 2 –2x³y.