c) C = 4(x + 1)2 + (2x – 1)2 – 8(x – 1)(x + 1) – 4x.

c) C = 4(x + 1)2 + (2x – 1)2 – 8(x – 1)(x + 1) – 4x.

= [2(x + 1)]2 + (2x – 1)2 – 8(x2 – 1) – 4x

= [2(x + 1)]2 + (2x – 1)2 – 8x2 + 8 – 4x

= [2(x + 1)]2 + (2x – 1)2 – 8x2 – 8x + 4x + 4 + 4

= [2(x + 1)]2 + (2x – 1)2 – 8x(x + 1) + 4(x + 1) + 4

= [2(x + 1)]2 + (2x – 1)2 + (x + 1)(4 – 8x) + 4

= [2(x + 1)]2 + (2x – 1)2 – 4(x + 1)(2x – 1) + 4

= [2(x + 1)]2 + (2x – 1)2 – 2 . 2(x + 1)(2x – 1) + 4

= [2(x + 1) – (2x – 1)]2 + 4

= (2x + 2 – 2x + 1)2 + 4

= 32 + 4 = 9 + 4 = 13.

Vậy biểu thức C không phụ thuộc vào giá trị của biến.