A parallelogram in which the diagonals are equal AND perpendicular must be a:

A parallelogram in which the diagonals are equal AND perpendicular must be a:
  1. Rectangle
  2. Rhombus
  3. Square
  4. Trapezium

Solution

Equal diagonals in a parallelogram make it a rectangle; perpendicular diagonals in a parallelogram make it a rhombus. Both together force a square.

Asked in: IMO

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