(2a+b-c+3d)(2a-b+c+3d)
=(2a+3d+b-c)(2a+3d-b+c)
=(2a+3d^)2-(b-c)^2
=4a^2+12ad+9d^2-b^2-c^2+2bc
[3x-2y+z][3x-2y-z] =（3x-2y)^2-z^2=9x^2-12xy+4y^2-z^2
(X-3)(X+2)=X^2-X-6
不懂的欢迎追问,