x^2+3x-4十字相乘法是把x^2的系数当成1x1常数项当成-1x41-1x14使得交叉相乘=x项系数3x^2+3x-4=(x-1)(x+4)6x^2-x-12-1X316x^2-x-1=(2x-1)(3x+1)
