Which expression is a trinomial?

Which expression is a trinomial?
  1. $x^{2} + 2xy + y^{2}$
  2. $3xyz$
  3. $x + y$
  4. $a + b + c + d$

Solution

A trinomial has exactly three unlike terms. $x^{2} + 2xy + y^{2}$ has three terms.

Asked in: IMO

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