A question about errors!

[Cliodhnqa]

Hello! 
Can anyone tell me how to calculate the error of the following. I have an equation that is ln or log X. X is calculated from the following equation:
ln [A/(A-B)]

A & B have both an error. It's so long since I've done any of these stats calculation...I'd be most grateful if anyone can help me out. Thanks a lot.
S.
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