Question

已知关于XY的方程组{X-Y=A+3,2X+Y=5A,若X>Y,求A的取值范围

Answer 1

x-y=a+3-----------------(1)

2x+y=5a-----------(2)

(1)+(2),3x=6a+3

x=(6a+3)/3

(1)*2,2x-2y=2a+6-----------(3)

(3)-(2),-3y=-3a+6,

y=(-3a+6)/-3

X>Y

(6a+3)/3>(-3a+6)/-3

3(-3a+6)>-3(6a+3)

-9a+18>-18a-9

-9a+18a>-9-18

9a>-27

a>-3