∵a>0 ,且x>1
∴(x2-2x+a+2)/(x-1)
=(x-1)+[(a+1)/(x-1)]
》2·[(x-1)·(a+1)/(x-1)]^(1/2)
= 2·(a+1)^(1/2)最小值为4
∴解得a = 3