b^2+c^2=(m^2-n^2)^2+(2mn)^2=m^4-2m^2n^2+n^4+4m^2n^2=m^4+2m^2n^2+n^4
=(m^2+n^2)^2=a^2
即: a^2=b^2+c^2
由勾股定理,该三角形一定是直角三角形。a为斜边。