b) (x + y – 4)(x – y + 4).

b) Cách 1:

(x + y – 4)(x – y + 4)

= x(x – y + 4) + y(x – y + 4) – 4(x – y + 4)

= x² ‒ xy + 4x + xy ‒ y² + 4y ‒ 4x + 4y ‒ 16

= x² ‒ y² +(‒xy + xy) + (4x ‒ 4x) + (4y + 4y) ‒ 16

= x² ‒ y² + 8y ‒16.

Cách 2:

(x + y – 4)(x – y + 4)

= [x + (y – 4)].[x – (y – 4)]

= x² – (y – 4)²

= x² – (y² – 8y + 16)

= x² ‒ y² + 8y ‒16.