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GATE 2025 ST – Question 47

Calculus · Sequences and Series · 2 marks · Multiple select

For n≥1, x_n=1+1/√2+1/√3+⋯+1/√n−2(√n−1). Which hold?

  1. Unbounded
  2. Monotonically decreasing
  3. Bounded but nonconvergent
  4. Convergent

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Show answer and explanation

Correct answer: (B) Monotonically decreasing; (D) Convergent

Explanation

The increment is 1/√(n+1)−2/[√(n+1)+√n]<0, so the sequence decreases. Integral comparison gives Σₖ₌₁ⁿ1/√k≥∫₁ⁿt^(−1/2)dt=2(√n−1), hence x_n≥0; also x_n≤x₁=1. A bounded monotone sequence converges. **B,D**.

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