ç´°ã輪ã®é¢ç©ãæ±ãããã 輪ã¯ãä¸å¿ãåãããã大ããåã¨å°ããåã¨ã«æã¾ããé åã¨è¨ããã®ã§ã大ããåã®åå¾ããå°ããåã®åå¾ãã¨ããã¨ãé¢ç©ã¯ä»¥ä¸ã®å¼ã§è¡¨ããã \begin{equation} S=\pi (r+dr)^2-\pi r^2 \end{equation} å¼ãå±éããã \begin{eqnarray} \require{cancel} S&=&\pi (\cancel{r^2}+2rdr+dr^2)-\cancel{\pi r^2} \end{eqnarray} ããã§è¼ªãç´°ããã¨ã¯ãå°ãããã¨ã¨ç価ã§ãããããããã®æãã®é ã¯ããã«å¤§å¹ ã«å°ããå¤ã«ãªãã®ã§ç¡è¦ã§ããã \begin{eqnarray} \require{cancel} S&=&\pi(2rdr+\cancel{dr^2})\\ &=&2\pi r dr \end{eqnarray} çµå±ãã®å¤ã¯åå¨
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