a^2-a^4=a^2(-a^2-1)=a^2(a^2-1)=-[a^2-1+1](a+1)(a-1)=-[(a-1)(a+1)+1]（a-1）(a+1)=-[(a-1)^2(a+1)^2+(a+1)(a-1)]把（a+1）(a-1)提出来=-{[(a+1)(a-1)](a+1)(a-1)+1}
a^2-a^4＝(a+a²)(a-a²)＝(a+a²)(√a+a)(√a-a)
