已知函數(shù)f(x)=log a (mx-1)/(1-x) (a>0,a≠1,m≠1)是奇函數(shù).
(1)求實數(shù)m的值;
(2)當a>1時,判斷函數(shù)f(x)在(1,+無窮)上的單調性,并給出證明.
(1)f(-x)=loga#[(-mx-1)/(1+x)]=-f(x)=loga#[(1-x)/(mx-1)]=loga#[(x-1)/(1-mx)],故m=-1
(2)f(x)=loga#[(x+1)/(x-1)],(x+1)/(x-1)>0,即x>1或x<-1.在x>1時為減函數(shù),證明如下:
當a>1時,設1
=loga#[(x2+1)(x1-1)/(x1+1)(x2-1)],因[(x2+1)(x1-1)/(x1+1)(x2-1)]-1=2(x1-x2)/(x1+1)(x2-1)<0
即(x2+1)(x1-1)/(x1+1)(x2-1)<1,故f(x2)-f(x1)
1時f(x)是減函數(shù).