The GATE Grind

GATE 2024 MA – Question 14

Real Analysis · Measure and Integration · 1 mark · Multiple choice

Evaluate the source limit $\lim_{\varepsilon\to0}\varepsilon^{-1}\int_0^\infty e^{-x/\varepsilon}[\cos3x+x^2+\sqrt{x+4}]dx$.

  1. Does not exist
  2. Exists and equals 0
  3. Exists and equals 3
  4. Exists and equals π

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Correct answer: (C) Exists and equals 3

Explanation

The improper integral is defined for positive ε; the intended limit is ε↓0. Put x=εt to get $\int_0^\infty e^{-t}[\cos(3\varepsilon t)+\varepsilon^2t^2+\sqrt{4+\varepsilon t}]dt$. Dominated convergence for 0<ε≤1 gives (1+0+2)∫e⁻ᵗdt=**3, C**. The source omits the positive-side qualifier; for negative ε the improper integral diverges.

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