a=m^2+n^2
b=m^2-n^2
c=2mn
b^+c^2=(m^2-n^2)^2+(2mn)^2
=m^4-2m^2*n^2+n^4+4m^2*n^2
=m^4+2m^2*n^2+n^4=(m^2+n^2)=a^2
即：b^2+c^2=a^2
所以,m^2+n^2,m^2-n^2,2mn這三個(gè)數(shù)就是一組勾股數(shù)組.