（m^2n^2+1)^1(这里是1次方么?不对吧?是2次方吧?我就当2次方算了)-4m^2n^2
（m^2n^2+1)^2-4m^2n^2
=(m^2n^2+2mn+1)(m^2n^2-2mn+1)
=(mn+1)^2(mn-1)^2