3x²+5xy-2y²+x+9y-4
=3x²+(5y+1)x-(2y²-9y+4)
=3x²+(5y+1)x-(y-4)(2y-1)
=(3x-y+4)(x+2y-1)-----十字交叉相乘法
3x²+5xy-2y²+x+9y-4
=(x+2y)(3x-y)+x+9y-4
=(x+2y)(3x-y)+(4x+8y-3x+y)-4
=(x+2y)(3x-y)+4(x+2y)-(3x-y)-4
=(x+2y)(3x-y) -(3x-y)+4(x+2y) -4
=(x+2y-1)(3x-y)+4(x+2y-1)
=(x+2y-1)(3x-y+4)记得采纳哦