f(x)在(0,1]上是增函数,在[1,+∞)是减函数.证明如下：f(x1)-f(x2)=x1/(x1^2+1)-x2/(x2^2+1)=[x1(x2^2+1)-x2(x1^2+1)]/[(x1^2+1)(x2^2+1)]=[x1x2(x2-x1)+x1-x2]/[(x1^2+1)(x2^2+1)]=(x2-x1)(x1x2-1)/[(x1^2+1)(x2^2+1...