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GATE 2021 MA – Question 15

Ordinary Differential Equations · Stability and Eigenvalue Problems · 1 mark · Multiple choice

For y″+λy=0,0<x<π,λ>0, with y(0)=0 and y(π)−y′(π)=0, which are the eigenvalues?

  1. λ=(nπ)²,n=1,2,…
  2. λ=n²,n=1,2,…
  3. λ=k_n², where k_n are the positive roots of k−tan(kπ)=0
  4. λ=k_n², where k_n are the positive roots of k+tan(kπ)=0

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Correct answer: (C) λ=k_n², where k_n are the positive roots of k−tan(kπ)=0

Explanation

Set k=√λ>0. The first condition gives y=C sin(kx). The second is C[sin(kπ)−k cos(kπ)]=0. For C≠0, cos(kπ) cannot vanish, so tan(kπ)=k. Thus **C**; the zero root is excluded by λ>0.

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