初中因式分解数学题十字相乘法a)x^2-y^2+5x+3y+4b)xy+y^2+x-y-2十字相乘法因式分解..注意:＾2表示2次方

a) 先按x降幂排列

a)x^2-y^2+5x+3y+4
=（x-y）(x+y)+4(x+y)+x-y+4
=(x+y)(x-y+4)+x-y+4
=(x+y+1)(x-y+4)
b)xy+y^2+x-y-2
=(xy+x)+(y^2-y-2)
=x(y+1)+(y-2)(y+1)
=(y+1)(x+y-2)