# Rational calculator

I am calculating a recursive equation: $X_n = \frac{1}{2}(X_{n-1} + \frac{3}{X_{n-1}})$

I need a free online calculator that can calculate for $n=7$

$(1/2)\times(88063572/50843527+(3\times 50843527)/88063572)$

and give the answer in rational form. The ones I've tried online do not work as the numbers get too big for the calculators. Wolfram alpha also does not work as it gives me the wrong answer of $X_7 =88063572/50843527$, which is same as $X_6$, so it is wrong.

• Is the last -1 in the index or not? Dec 6 '13 at 0:14
• Just as a side note, you can also find a closed form for $X_n$, using the coth function. Dec 6 '13 at 0:34
• @arbautjc: This is Newton's method for $\sqrt 3$ Dec 6 '13 at 0:57
• Minor correction: $X_n=\sqrt{a} \coth\left(2^n \arg \coth \frac{X_0}{\sqrt{a}}\right)$. And if $0 \leq X_0 < \sqrt{a}$, you have anyway $X_i>\sqrt{a}$ for all $i>0$, so you can still use the formula. Dec 6 '13 at 10:57

I put it into Alpha and got $\frac{5170128475599457}{2984975067132296}$
• If you got exactly $X_6$ I would suspect a copy/paste error, where you thought you were asking for $X_7$ and instead asked for $X_6$. I know I have done that. Just a guess. Works fine for me that way Dec 6 '13 at 0:18