a=m^2-n^2,a^2=n^4+m^4-2m^2n^2
b=m^2+n^2,b^2=m^4+n^4+2m^2n^2
c=2mn,c^2=4m^2n^2
a^2+c^2=m^4+n^4+2m^2n^2=b^2
三角形为直角三角形