（x+2）（x-2）-（x-2）^2=（x-2）[（x+2）-（x-2）]=4(x-2)=4x-8（x-y-1）^2=[（x-y)-1）]^2=（x-y)^2+1-2(x-y)=x^2+y^2-2xy+1-2x+2y=x^2+y^2-2xy-2x+2y+1（3a-b+c）（3a+b-c）=[3a-(b-c)][3a+(b-c）]=9a^2-(b-c)^2...