Factorise $x^{2} - 4x - 21$.

Factorise $x^{2} - 4x - 21$.
  1. $(x - 7)(x + 3)$
  2. $(x + 7)(x - 3)$
  3. $(x - 21)(x + 1)$
  4. $(x - 7)(x - 3)$

Solution

Need $a+b=-4, ab=-21$: $a=-7, b=3$. So $(x-7)(x+3)$.

Asked in: IMO

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