The figure shows a parallelogram ABCD. If squares of area S₁, S₂, S₃, and S₄ are constructed on the sides and diagonals, prove that 2S₁ + 2S₂ = S₃ + S₄ . View or post a solution.

See also: 

  Median length, Apollonius' Theorem
  Pythagoras Theorem
  Euclid's Elements Book II Proposition 13
  Stewart Theorem