x^2 + 4y^2 +4z^2 + 4xy - 4xz - 8yz
=x^2 +(2y)^2 +(2z)^2 + 2*x*(2y) -2x*(2z) - 2 * (2y)*(2y)
=( x-2y -2z)^2
利用公式：(a+b+c)^2 = a^2 +b^2 +c^2 +2ab +2ac+2bc