x^2+3(x+y)+3-y^2+(x-y)

x^2+3(x+y)+3-y^2+(x-y)
=x^2+4x+2y+3-y^2
=(x^2+4x+4)-(y^2-2y+1)
=(x+2)^2-(y-1)^2
=(x+y+1)(x-y+3)
本题也可用双十字相乘法更简单，只不过一般人没学。

化简
X^2+3(X+Y)+3-Y^2+(X-Y)
=X^2-Y^2+3(X+Y)+(X-Y+3)
=(X+Y)(X-Y)+3(X+Y)+(X-Y+3)
=(X+Y)(X-Y+3)+(X-Y+3)
=(X-Y+3)(X+Y+1)