f(x)是偶函数,
f(-x)=-x[1/(2^(-x)-1)+1/2]
=-x( 2^x/(1-2^x) + 1/2 )
=x( 2^x/(2^x-1) - 1/2 )
f(x)-f(-x)=x( (1-2^x)/(2^x-1) + 1/2 - (-1/2) )
=x(-1+1)=0
所以f(x)=f(-x)