Question

1.已知X、Y、Z、是3个非负数,满足3x+2y+z=5,x+y-z

Answer 1

(1)

x+y-z=2.

y-z=2-x

所以S=2X+Y-Z=X+2

3x+2y+z=5,

x+y-z=2.

两个式子加起来4X+3Y=7 X=(7-3Y)/4

Y≥0所以0≤X≤1

所以2≤S≤3

2、

{xy+x+y+7=0 (1)

{3x+3y=9+2xy(2)

由(1)得

x+y=-7-xy

代入（2）

3(-7-xy)=9+2xy

-21-3xy=9+2xy

5xy=-30

xy=-6

那么x+y=-7-(-6)=-1

{xy=-6

{x+y=-1

解得x=-3 y=2

x^2+xy^2=(-3)²+(-3)*2²=9-12=-3