1：相加
4x 4y=0
x=-y
代入
z=3y
2x^2 y^2/x^2 y^2 z^2
=3y^2/11y^2
=3/11
…
2：x y z
=a^2-2b 1 b^2-2c 2 c^2-2a 3
=(a-1)^2 (b-1)^2 (c-1)^2
则x y z的取值范围是大于等于0