((a ⋅ b) ⋅ c) ⋅ d = ((a ⋅ c) ⋅ b) ⋅ d

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• ((a ⋅ b) ⋅ c) ⋅ d = ((a ⋅ c) ⋅ b) ⋅ d,

  if a = b, then a ⋅ c = b ⋅ c

  • (a ⋅ b) ⋅ c = (a ⋅ c) ⋅ b, (a ⋅ b) ⋅ c = (a ⋅ c) ⋅ b