回復 1# bugmens 的帖子
題目 \( \displaystyle \sum\limits _{n=1}^{\infty}\frac{4^{2^{n-1}}}{4^{2^{n}}-1} \)
解 令 \( x_{n}=4^{2^{n}} \),利用 \( \displaystyle \frac{x_{n}}{x_{n+1}-1}=\frac{1}{2}\cdot\frac{1}{x_{n}+1}+\frac{1}{2}\cdot\frac{1}{x_{n}-1} \), \( \frac{1}{x_{n}-1}=\frac{1}{x_{n-1}^{2}-1}=-\frac{1}{2}\cdot\frac{1}{x_{n-1}+1}+\frac{1}{2}\cdot\frac{1}{x_{n-1}-1} \),可得以下:
\(\displaystyle \frac{4^{1}}{4^{2}-1}=\frac{1}{2}\cdot\frac{1}{4^{1}+1}+\frac{1}{2}\cdot\frac{1}{4-1} \)
\(\displaystyle \frac{4^{2}}{4^{4}-1}=\frac{1}{2}\cdot\frac{1}{4^{2}+1}-\frac{1}{4}\cdot\frac{1}{4+1}+\frac{1}{4}\cdot\frac{1}{4-1} \)
\(\displaystyle \frac{4^{4}}{4^{8}-1}=\frac{1}{2}\cdot\frac{1}{4^{4}+1}-\frac{1}{4}\cdot\frac{1}{4^{2}+1}-\frac{1}{8}\cdot\frac{1}{4+1}+\frac{1}{8}\cdot\frac{1}{4-1} \)
\(\displaystyle \frac{4^{3}}{4^{16}-1}=\frac{1}{2}\cdot\frac{1}{4^{8}+1}-\frac{1}{4}\cdot\frac{1}{4^{4}+1}-\frac{1}{8}\cdot\frac{1}{4^{2}+1}-\frac{1}{16}\cdot\frac{1}{4+1}+\frac{1}{16}\cdot\frac{1}{4-1} \)
...
斜的加,從左上往右下加,把每一條斜線加總可得
\( \displaystyle \sum\limits _{n=1}^{m}\frac{4^{2^{n-1}}}{4^{2^{n}}-1}=\frac{1}{2^{m+1}}\sum\limits _{n=1}^{m}\left(\frac{2^{n}}{4^{2^{n-1}}+1}\right)+\frac{2^{m}-1}{2^{m}}\cdot\frac{1}{3}\to\frac{1}{3} \)
題目:\( (\sqrt{23}+\sqrt{27})^{100} \) 除以 100 的餘數,這題應該加上高斯符號
解 \( (\sqrt{23}+\sqrt{27})^{100}=\left(50+6\sqrt{69}\right)^{50} \)。
令 \( x_{n}=\left(50+6\sqrt{69}\right)^{50}+(50-6\sqrt{69})^{50} \),則 \( x_{n+2}=100x_{n+1}-16x_{n}
\Rightarrow x_{50}\equiv x_{0}\cdot(-16)^{25}\equiv-2^{101} (Mod 100) \)。
\( \phi(25)=20 \Rightarrow x_{50}\equiv-2 (Mod 25), x_{50}\equiv0 (Mod 4) \),故 \( x_{50}\equiv48 (Mod 100) \)。
注意 \( 0<(50-6\sqrt{69})^{50}<1 \),故 \( x_{50}=\left(50+6\sqrt{69}\right)^{50}+(50-6\sqrt{69})^{50}=\left[\left(50+6\sqrt{69}\right)^{50}\right]+1 \)。
因此 \( \left[(\sqrt{23}+\sqrt{27})^{100}\right]\equiv47 (Mod 100) \)。
[ 本帖最後由 tsusy 於 2014-4-13 11:14 PM 編輯 ]