x＞1
y = log 1/2 {x+1/(x-1)-1)}
= log 1/2 {(x-1) +1/(x-1)}
∵x-1＞0
∴(x-1) +1/(x-1) = {根号(x-1) - 1/根号(x-1) }^2 + 2 ≥ 2
∵底数1/2 ＜1
∴当真数(x-1) +1/(x-1)取最小值时,y取最大值
∴ymax = log 1/2 2 = -1