Which expression is a binomial?

Which expression is a binomial?
  1. $3x$
  2. $x^{2} + 4x + 4$
  3. $5y^{2} - 7$
  4. $a + b + c + d$

Solution

A binomial has exactly two unlike terms. $5y^{2} - 7$ has two terms.

Asked in: IMO

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