=[（a^2+1）+2a][(a^2+1)-2a]平方差公式=(a^2+1)^2-(2a)^2=a^4+2a^2+1-4a^2=a^4-2a^2+1=(a^2-1)^2=(a-1)^2(a+1)^2
=(a+1)^2(a-1)^2
