x-y=a+3 1
2x+y=5a 2
1+2得 x-y+2x+y=a+3+5a 3x=6a+3 x=2a+1
2-1X2得 2x+y-2x+2y=5a-2a-6 3y=3a-6 y=a-2
∵x＞0 y＞0
∴2a+1＞0 a-2＞0
∴a＞-1/2 a＞2
∴a＞2