1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Iterative formulas

  1. Mar 5, 2009 #1
    1. The problem statement, all variables and given/known data

    First of all this is revision not homework =)

    question is Xn+1 = Cuberoot ( 17.5-2xn)

    answer lies between 2 and 3. i know the answer is 2.34 but what i don't get is why it is
    xn+1 = equation, because when you put xn=2.34 into the equation you get 2.34 out.
    shouldnt get 3.34 out if its xn+1. whats the purpose of the n+1!!
  2. jcsd
  3. Mar 5, 2009 #2
    xn and xn+1 both go (in this convergent case) to a limit call it L. Now substitute L for xn and xn+1 and solve the equation.
  4. Mar 5, 2009 #3
    i don't think you understand what i'm confused with,

    i know the answer is 2.34 so i put it in to check

    xN+1 = cuberoot(17.5-2xn)


    do you understand what i dont understand.
  5. Mar 5, 2009 #4
    You've made an error in your calcution it is correct.
  6. Mar 5, 2009 #5
    If you mean


    then the answer is [tex]\lim_{n\to\infty}x_n=2.34[/tex], and this solution can be found as dirk_mec1 described.

    But if you mean


    then [tex]x_n=1.44[/tex] for all [tex]n[/tex].
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook