（a+b）²-1=（a+b+1)(a+b-1)4a²-(a²+1)²=(2a+a²+1)(2a-a²-1)=-(a+1)²(a-1)²4（x+y）²-25=[2(x+y)+5][2(x+y)-5]=(2x+2y+5)(2x+2y-5)x（x+y）（x-y）-x（x+y）²x（x+y）（x-y）-x（x+y）²

=x(x+y)[(x-y)-(x+y)]
=-2xy(x+y)