Cách 1:
Ta có: x2 – x – 12 = x2 – x – 16 + 4
= (x2 – 16) – (x – 4)
= (x – 4).(x + 4) – (x – 4)
= (x – 4).(x + 4 – 1)
= (x – 4).(x + 3)
Cách 2:
x2 – x – 12 = x2 – x – 9 – 3
= (x2 – 9) – (x + 3)
= (x – 3).(x + 3) – (x + 3)
= (x + 3).(x – 4)
Cách 3:
x2 – x – 12 = x2 – 4x + 3x – 12
= (x2 – 4x) + (3x – 12)
= x.(x – 4) + 3.(x – 4)
= (x – 4).(x + 3).
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