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GATE 2020 MA – Question 20

Numerical Analysis · Nonlinear Equations · 1 mark · Multiple choice

For $x_0>0$, define $x_n=x_{n-1}/2+3/x_{n-1}$ for n ≥ 1. The sequence

  1. Converges only if x₀ > 1.
  2. Converges only if x₀ < 3.
  3. Converges for every x₀ > 0.
  4. Does not converge for any x₀.

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Correct answer: (C) Converges for every x₀ > 0.

Explanation

This is Newton iteration for $x^2-6=0$. For every positive x, AM–GM gives $x/2+3/x\geq\sqrt6$. Thus x₁ ≥ √6. Once $x_n\geq\sqrt6$, $x_{n+1}-x_n=(6-x_n^2)/(2x_n)\leq0$, so the sequence decreases and is bounded below. Its positive limit solves $L=L/2+3/L$, hence L = √6. Therefore C.

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