b) (a – 4b)(a + 2b) + a(a + 2b).

b) Cách 1:

(a – 4b)(a + 2b) + a(a + 2b)

= a(a + 2b) – 4b(a + 2b) + a² + 2ab

= a² + 2ab – 4ab – 8b² + a² + 2ab

= (a² + a²) + (2ab – 4ab + 2ab) – 8b²

= 2a² – 8b².

Cách 2:

(a – 4b)(a + 2b) + a(a + 2b)

= (a + 2b)(a ‒ 4b + a)

= (a + 2b)(2a ‒ 4b)

= 2(a + 2b)(a ‒ 2b)

= 2[a² ‒ (2b)²] = 2(a² – 4b²)

= 2a² ‒ 8b².