Cho x + y = 3. Tính giá trị biểu thức:

A = x³ + x²y – 3x² + xy + y² – 4y – x + 3.

Ta có A = x³ + x²y – 3x² + xy + y² – 4y – x + 3.

= (x³ + x²y – 3x²) + (xy + y² – 3y) – (x + y – 3).

= x²(x + y – 3) + y(x + y – 3) – (x + y – 3).

= (x + y – 3)(x² + y – 1).

= (3 – 3)(x² + y – 1).

= 0.(x² + y – 1).

= 0.

Vậy A = 0 khi x + y = 3.