1-2m^2+m^4
=（1-m^2)^2
=(1-m)^2(1+m)^2为什么会等于（1-m^2）^2?完全平方式子a^2-2ab+b^2=(a-b)^2 1-2m^2+m^4=1^2-2*1*m^2+(m^2)^2=(1-m^2)^2