-loga(x+y/x-y)=loga(x+y/x-y)-1=loga(x-y/x+y)
注:负的log以a为底(x+y/x-y)的对数=log以a为底(x+y/x-y)的负一次方的对数=log以a为底(x-y/x+y)的对数