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ABCD be parallelogram. The parallelogram law states : sum of squares of sides = sum of squares of diagonals.  a, b are sides. c, d are diagonals.    here c = d.
   2 a² + 2 b² = c² + d² = 2 c²    =>    a² + b² = c²
  => a and b are at right angles. => rectangle.