解(x-y-1)²=[(x-y)-1]²=(x-y)²-2(x-y)+1=x²-2xy+y²-2x+2y+1(3a-b+c)(3a+b-c)=[3a-(b-c)][3a+(b-c)]=(3a)²-(b-c)²=9a²-b²+2ab-c²
