( I)函数定义域为R,
∵f(x)=
(ax?a?x) (a>0且a≠1).a
a2?1
∴f(?x)=
(a?x?ax)=?f(x),a
a2?1
∴f(x)为奇函数.
( II)设x1,x2∈R且x1<x2,
∴f(x1)?f(x2)=
[(ax1?a?x1)?(ax2?a?x2)]=a
a2?1
(ax1?ax2+a?x2?a?x1)=a
a2?1
(ax1?ax2+a
a2?1
?1 ax2
)1 ax1
=
(ax1?ax2+a
a2?1
)=
ax1?ax2
ax1+x2
(ax1?ax2)(1+a (a+1)(a?1)
),1 ax1+x2
当a>1时,
>0,ax1?a
a (a+1)(a?1)