x²-(p²+q²)x+pq(p+q)(p-q﹚＝[x－q﹙p＋q﹚][x－p﹙p－q﹚]
x²-2xy-8y²-x-14y-6＝﹙x＋2y＋2﹚﹙x－4y－3﹚
交叉相乘法.