﹙x＋y＋z﹚²－﹙x＋y－z﹚²
=[﹙x＋y＋z﹚－﹙x＋y－z﹚][﹙x＋y＋z﹚+﹙x＋y－z﹚]
=(2z)(2x+2y)
=4xz+4yz