Tosfas asks, PI does not agree with the knowledge of measurements? Maybe that is the kashye of the gemora on the mishna in Eruvin, if the diameter is a tefach than the circumference is three tefochim, so the gemora asks מנא הני מילי, how do we know that? What does that mean when one can measure it? The question is, how do we know that we a can approximate it like that? So to understand a gemora, math is required as the Rambam on the mishna explains it well.
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