Question

已知x-2y+z=0,3x+6y-z=0(xyz≠0),求2x²+y²/x²+y²

Answer 1

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