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.

  • 1
    $\begingroup$ Is the last -1 in the index or not? $\endgroup$ Dec 6 '13 at 0:14
  • $\begingroup$ Just as a side note, you can also find a closed form for $X_n$, using the coth function. $\endgroup$ Dec 6 '13 at 0:34
  • $\begingroup$ @arbautjc: This is Newton's method for $\sqrt 3$ $\endgroup$ Dec 6 '13 at 0:57
  • $\begingroup$ 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. $\endgroup$ Dec 6 '13 at 10:57

I put it into Alpha and got $\frac{5170128475599457}{2984975067132296}$

  • $\begingroup$ Didn't work for me when I entered it in the format (1/2)×(88063572/50843527+(3×50843527)/88063572) for some reason... Strange... Thanks anyways $\endgroup$
    – user110069
    Dec 6 '13 at 0:16
  • $\begingroup$ 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 $\endgroup$ Dec 6 '13 at 0:18
  • $\begingroup$ what i actually did was "is 0.5×(88063572/50843527+(3×50843527)/88063572) rational" for some reason this didnt work. But if i do "(1/2)×(88063572/50843527+(3×50843527)/88063572)" it works $\endgroup$
    – user110069
    Dec 6 '13 at 0:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.