Áp dụng HĐT mở rộng: (a + b + c)² = a² + b² + c² + 2ab + 2bc + 2ac
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3(x^4 + x^2 + 1) - (x^2 + x + 1)^2
= (3x^4 + 3x^2 + 3) - (x^4 + x^2 + 1 + 2x^3 + 2x^2 + 2x)
= 3x^4 + 3x^2 + 1 - x^4 - x^2 - 1 - 2x^3 - 2x^2 - 2x
= (3x^4 - x^4) + 2x^3 + (3x^2 - x^2 - 2x^2) - 2x + (1 - 1)
= 2x^4 + 2x^3 - 2x
@vietdorapan