Factorise $x^{2} + 7x + 12$.

Factorise $x^{2} + 7x + 12$.
  1. $(x + 3)(x + 4)$
  2. $(x + 2)(x + 6)$
  3. $(x - 3)(x - 4)$
  4. $(x + 1)(x + 12)$

Solution

Find $a,b$ with $a+b=7, ab=12$: $a=3, b=4$. So $x^{2}+7x+12=(x+3)(x+4)$.

Asked in: IMO

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