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#1
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͹ءÃÁàâҤ³ÔµÍ¹Ñ¹µì
àÃ×èͧ͹ءÃÁ͹ѹµìÍèФèÐ ËҼźǡÁѹ·ÓÂѧ䧤РªèÇÂ͸ԺÒÂÅÐàÍÕ´æ˹èͤèèÐ
$\sum_{n = 1}^{\infty}\frac{3}{4^{n-2}} $ $\sum_{n = 1}^{\infty} \frac{1}{2\cdot 5^n}$ ¢Íº¤Ø³¤èÒÒÒ
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Í´Õµ¤×ÍÀÒ¾¾¨¹ì ͹Ҥµ¤×ͤÇÒÁ½Ñ¹ »Ñ¨¨ØºÑ¹¤×ͤÇÒÁ¨ÃÔ§ |
#2
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ãªéÊٵà $S_{\infty }=\dfrac{a_1}{1-r}$ àÁ×èÍà»ç¹Í¹Ø¡ÃÁàâҤ³Ôµ â´Â $a_1$ ¤×;¨¹ì·Õè˹Öè§ $r$ ¤×ÍÍѵÃÒÊèǹÃèÇÁ
$$\sum_{n = 1}^{\infty}\frac{3}{4^{n-2}}=3\sum_{n = 1}^{\infty}\frac{1}{4^{n-2}}=3(\frac{1}{4^{-1}}+\frac{1}{4^0}+\frac{1}{4^1}+...)=3(\dfrac{\frac{1}{4^{-1}}}{1-\frac{1}{4}})=16$$ ¢éÍ·ÕèÊͧ·Óàªè¹à´ÕÂǡѹ ãªèËÃ×Íà»ÅèÒà¹Õè àËÍæ
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#3
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ÍéÒ§ÍÔ§:
$$ \frac{1}{2}*(\frac{1}{5}+\frac{1}{125}+.......)$$ $$\frac{1}{2}*(\frac{\frac{1}{5} }{1-\frac{1}{5} }$$ $$\frac{1}{8}$$
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Fortune Lady
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#4
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à¾ÔèÁàµÔÁ¤ÃѺ ¨Ò¡Êٵôѧ¡ÅèÒÇ ÊÓËÃѺ $r$ ¹Ñ鹨ÐÁÕ¤èÒ $\left|\,\right. {r}\left.\,\right| <1$ àÊÁͤÃѺ
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