newton's method for approximating zeros... blargh

Discussion in 'General Discussions' started by ceneret0023, Nov 20, 2009.

  1. ceneret0023

    ceneret0023 Level III

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    I had to miss the lesson for this in my math class cuz i had to take stupid standardized tests and i don't think i fully understand it...

    so i have to find two consecutive iterations that are withing .001 of eachother for the function

    f(x) = 3√(x-1) - x

    i got f'(x) = (3-2√(x-1))/(2√(x-1)) as the derivative

    i started with x1 = 1 and got these iterations
    x2 = 1.5
    x3 = 1.189
    x4 = 1.131
    x5 = 1.153
    x6 = 1.140
    x7 = 1.149

    and i began to get confused because my iterations kept going in that pattern bigger than smaller and i got really confused so if somebody could please explain to me what maybe i'm doing wrong, and also is there any way to find a better x1 that can put me closer without actually solving the equation for the zeros cuz i really don't feel like doing 10 iterations of every function i have to solve lol
     
  2. Zer0

    Zer0 Level IV

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    The derivative is 3/(2sqrt(x-1)) - 1
    The starting value shouldn't matter too much (as long as you're not 100 away or something) since Newton's method converges quadratically.
     
  3. ceneret0023

    ceneret0023 Level III

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    ok so if the function has multiple zeros would that mean i have to do that multiple times to find the different zeros and start with different starting values?
     
  4. Zer0

    Zer0 Level IV

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    Yup.
     
  5. ceneret0023

    ceneret0023 Level III

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    ugh eff my life lol ok thanks for the help