Find the product $(x + 3)(x + 5)$.

Find the product $(x + 3)(x + 5)$.
  1. $x^{2} + 8x + 15$
  2. $x^{2} + 15x + 8$
  3. $x^{2} + 2x + 15$
  4. $x^{2} - 8x + 15$

Solution

$(x+a)(x+b) = x^{2} + (a+b)x + ab$ with $a = 3,\ b = 5$: $x^{2} + 8x + 15$.

Asked in: IMO

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