Estimate the number of iterations of Newton's method needed to find a root of f(x)=cos(x)−x to within 10^(−100).

Izabelle Lowery

Izabelle Lowery

Answered question

2022-10-12

Estimate the number of iterations of Newton's method needed to find a root of f ( x ) = cos ( x ) x to within 10 100 .

Answer & Explanation

Adalyn Pitts

Adalyn Pitts

Beginner2022-10-13Added 15 answers

The idea behind the reasoning is the quadratic convergence of Newton's algorithm (if the zero is simple).
When you are near the zero α, an iteration takes you from α + δ to
( α + δ ) f ( α + δ ) f ( α + δ ) α + δ f ( α ) δ + f ( α ) δ 2 2 f ( α ) + f ( α ) δ α + f ( α ) 2 f ( α ) δ 2 ,
so each step roughly doubles the number of correct digits.
If you start with approximately one correct digit, after seven steps, you have roughly 2 7 = 128 correct digits.

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