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Thank you so much! Replies here are all useful. I tried to reply, but somehow my internet browser didn't allow it. It just copied what I had wrote... Anyway thank you!
I am using Mathematica 8.0.1. I learned Mathematica by myself, and till yesterday I didn't need any feature of Mathematica that is related with precision. But today I have to calculate some complicated formula... and the result should be quite...