(m+n)^2-(2m-n)(2m+n)+(2m+n)^2
=（m+n)^2-(2m+n)[(2m-n)-(2m+n)]
=(m+n)^2-(2m+n)(-2n)
=m^2+2mn+n^2+4mn+2n^2
=m^2+6mn+3n^2