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GATE 2022 MA – Question 18

Numerical Analysis · Nonlinear Equations · 1 mark · Multiple choice

For x_n=x_(n−1)/2+1/x_(n−1),n≥1, which is true?

  1. Converges to √2 for every nonzero real x₀.
  2. Converges to √2 whenever x₀>√(2/3).
  3. Converges to √2 for every x₀∈(−1,1)∖{0}.
  4. Diverges for every nonzero x₀.

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Show answer and explanation

Correct answer: (B) Converges to √2 whenever x₀>√(2/3).

Explanation

This is Newton iteration for x²−2=0. For x₀>0, AM–GM gives x₁≥√2; thereafter √2≤x_(n+1)≤x_n, with limit L=L/2+1/L giving L=√2. Negative initial values remain negative and converge to −√2, refuting A and C. B supplies a positive starting value and is true. **B**.

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