m² - 2m/4-m² ＝m(m-2)/(2+m)(2-m)=m/2+mx²+4x+4/4+2x =(x+2)²/2(2+x)=(x+2)/212a³(y-x)²/27(x-y)=12a³(x-y)²/27(x-y)=4a³(x-y)/92.分式为0,则分子必为0.（x-2)²...