$(a + b)^{2} - (a - b)^{2}$ equals

$(a + b)^{2} - (a - b)^{2}$ equals
  1. $4ab$
  2. $2ab$
  3. $4(a^{2} - b^{2})$
  4. $0$

Solution

Expanding: $(a^{2} + 2ab + b^{2}) - (a^{2} - 2ab + b^{2}) = 4ab$.

Asked in: IMO

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