3x²+5xy-2y²+x+9y-4
用待定系数法做
因为3x²+5xy-2y²=(3x-y)(x+2y)
所以设3x²+5xy-2y²+x+9y-4=(3x-y+a)(x+2y+b)
=3x²+5xy-2y²+ax+2ay+3bx-by+ab
=3x²+5xy-2y²+(a+3b)x+(2a-b)y+ab
比较系数,得 a+3b=1,2a-b=9,ab=-4
解得 a=4,b=-1
所以3x²+5xy-2y²+x+9y-4=(3x-y+4)(x+2y-1)