$(a + b)^{3}$ expands to

$(a + b)^{3}$ expands to
  1. $a^{3} + 3a^{2}b + 3ab^{2} + b^{3}$
  2. $a^{3} + b^{3}$
  3. $a^{3} + 2ab + b^{3}$
  4. $a^{3} + 3ab + b^{3}$

Solution

Standard binomial cube identity.

Asked in: IMO

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