回復 1# bch0722b 的帖子
題目錯了吧，應該是 \( x = \displaystyle \frac{5^\frac15+5^{-\frac15}}{2} \)
\( x^2 -1 = \displaystyle \frac{5^\frac25+2+5^{-\frac25}}{4} -1 = \left ( \frac{5^\frac15-5^{-\frac15}}{2} \right )^2 \)
\( x+\sqrt{x^2 -1} = \frac{5^\frac15+5^{-\frac15}}{2} + \frac{5^\frac15-5^{-\frac15}}{2} = 5^\frac15 \)
\( (x+\sqrt{x^2 -1})^n = 5 \)
