x^3+2x^2-5x-6=x^3+x^2＋x²-5x-6=x²﹙x＋1﹚＋﹙x＋1﹚﹙x－6﹚=﹙x＋1﹚﹙x²＋x－6﹚=﹙x＋1﹚﹙x＋3﹚﹙x－2﹚
x³+2x²-5x-6=x³-2x²+4x²-8x+3x-6=x²(x-2)+4x(x-2)+3(x-2)=(x²+4x+3)(x-2)=(x+1)(x+3)(x-2)
