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GATE 2022 BM – Question 37

Engineering Mathematics · Differential Equations · 2 marks · Multiple choice

Solve dy/dx−y=cos x, with c an arbitrary constant.

  1. $y=(\sin x-\cos x)/2+ce^x$
  2. $y=(\sin x+\cos x)/2+ce^x$
  3. $y=(\sin x-\cos x)/2+ce^{-x}$
  4. $y=(\sin x+\cos x)/2+ce^{-x}$

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Correct answer: (A) $y=(\sin x-\cos x)/2+ce^x$

Explanation

The homogeneous solution is ceˣ. Seek a particular solution a sin x+b cos x. Substitution gives (a−b)cos x+(−a−b)sin x=cos x, hence a=1/2,b=−1/2. Adding homogeneous and particular parts gives A. Differentiating A verifies the equation directly.

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