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