Đáp án:
Cách `1`:
`(x - 1)(x^3 + x^2 + x + 1)`
`= x^4 + x^3 + x^2 + x - x^3 - x^2 - x - 1`
`= x^4 + (x^3 - x^3) + (x^2 - x^2) + (x - x) - 1`
`= x^4 - 1`
Cách `2`:
`(x - 1)(x^3 + x^2 + x + 1)`
`= (x - 1)[(x^3 + x^2) + (x + 1)]`
`= (x - 1)[x^2(x + 1) + (x + 1)]`
`= (x - 1)(x + 1)(x^2 + 1)`
`= (x^2 - 1)(x^2 + 1)`
`= (x^2)^2 - 1^2`
`= x^4 - 1`
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