m-n=(a+1)^2(a-1)^2-(a^2+a+1)(a^2-a+1)=(a^2-1)^2-(a^2+1)^2+a^2=-4a^2+a^2=-3a^2为什么=(a+1)^2(a-1)^2等于(a^2-1)^2(a+1)^2(a-1)^2=[(a+1)(a-1)]^2=(a^2-1)^2