解:设-1<x1<x2<1,
则f(x1)-f(x2)=
=
=.
∵-1<x1<x2<1,
∴x2-x1>0,x1x2+1>0,(x12-1)(x22-1)>0.
又a>0,
∴f(x1)-f(x2)>0,函数f(x)在(-1,1)上为减函数.