Which one of the following is NOT necessarily true for every parallelogram?

Which one of the following is NOT necessarily true for every parallelogram?
  1. Opposite sides are parallel
  2. Opposite angles are equal
  3. Diagonals are equal
  4. Diagonals bisect each other

Solution

Diagonals are equal only in special parallelograms (rectangle and square). In a general parallelogram they are not equal.

Asked in: IMO

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