M=a/(a+1)-b/(b-1),N=1/(a+1) + 1/(b+1)
那么,
M-N
=a/(a+1)-b/(b-1)-[1/(a+1) + 1/(b+1)]
=a/(a+ab)-b/(b-ab)-1/(a+1)-1/(b+1)
=1/(1+b)-1/(1-a)-1/(a+1)-1/(b+1)
=1/(a-1)-1/(a+1)
=[(a+1)-(a-1)]/[(a-1)(a+1)]
=2/[(a-1)(a+1)]
当a<-1或a>1时,(a-1)(a+1)>0
M>N
当-1<a<1时,(a-1)(a+1)<0
M<N
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