原式为：p²(x＋y)²－q²(x－y)²
=[p(x＋y)]²－[q(x－y)]²
=[p(x＋y)＋q(x－y)][p(x＋y)－q(x－y)]
=(px＋py＋qx－qy)(px＋py－qx＋qy)