2x^2+5xy－3y^2-3x-16y-5=(2x-y)(x+3y)-3x-16y-5 =(2x-y)(x+3y)+(2x-y-5)-5x-15y =(2x-y)(x+3y)-5(x+3y)+(2x-y-5) =(x+3y)(2x-y-5)+(2x-y-5) =(2x-y-5)(x+3y+1) 这叫双十字相乘法