1、解方程组{x+y=3a+7
x-y=5a+1
得{x=4a+4
y=-a+3
∵x＞0,y＞0
∴{4a+4＞0
-a+3＞0
解得：-1＜a＜3
2、∵-1＜a＜3
∴a+1＞0,a-3＜0
∴│a+1│+│a-3|
=a+1+[-(a-3)]
=a+1-(a-3)
=a+1-a+3
=4