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$f(x)=-x^3-2x^2-4x-25$ this is the function, and I found the derivative $f' (x)=-3x^2-4x-4$, and put it into Newton's method. I let the first term to be -4, and i got

$x_(n+1)=-4+ 23/36 = -3.63....$

but from the website it sats the first term is -4.06...... it cant be. Everything is right. what is the problem?

from wolframalpha the actual answer is about 3.19

Tommy Lassa
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2 Answers2

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The web page is incorrect and you are correct. After five iterations I get the value $-3.19597980081$ on my TI-Nspire CX. I also get the first iteration value $\frac{-121}{36}\approx -3.361111111111$ (you made a mistake in the decimal approximation).

Rory Daulton
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  • Thank you :) In that case, do you know any online websites that can give me accurate solutions for Newton's method? As I am tempted to use my calculator but I need more digits ( about 22 digits), which cannot be shown in calculator. – Tommy Lassa Nov 29 '15 at 12:21
  • The standard Windows calculator handles 32 digits. –  Nov 29 '15 at 12:23
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If you give the website input complete with multiplications signs,

f(x)  : -x^3-2*x^2-4*x-25
f'(x) : -3*x^2-4*x-4

then the expression parser gets the correct function and convergence in 7 steps.

Lutz Lehmann
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