原式=(x+2y)方+6(x+2y)+9-((x-2y)方-9) =6(x+2y)+18 =6x+12y+18
(x+2y+3)^2 - (x-2y+3) * (x-2y-3)=(x+2y+3)^2 - (x-2y)^2+9=(x+2y+3+x-2y)*(x+2y+3-x+2y)+9=(2x+3)*(4y+3)+9=8xy+6x+12y+18
