(1)∵f(x)的定义域为R,设x1<x2,
则f(x1)-f(x2)=a-12x1+1-a+12x2+1=2x1-2x2(1+2x1)(1+2x2),
∵x1<x2,∴2x1-2x2<0,(1+2x1)(1+2x2)>0,∴f(x1)-f(x2)<0,
即f(x1)<f(x2),所以不论a为何实数f(x)总为增函数.
(2)∵f(x)为奇函数,∴f(-x)=-f(x),即a-12-x+1=-a+12x+1,
解得:a=12.∴f(x)=12-12x+1.
(3)由(2)知f(x)=12-1手准调何促陈里2x+1(4),∵2x+1>1(5),∴0<12x+1<1(6),∴-1<-12x+1<0,∴来自-12<f(x)<12
所以f(x)的值域为(-12,12).