已知,y = (a-x)/(x-a-1) = [-(x-a-1)-1]/(x-a-1) = -1-1/(x-a-1) ,
可得：y+1 = -1/(x-a-1) ,
则有：该函数图像的对称中心是 (a+1,-1) ,
依题意可得：a+1 = 3 ,
解得：a = 2 .