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For the linear equation \( \dfrac{dy}{dx} + Py = Q \) with \( P \) a function of \( x \), the integrating factor is

  • A.\( e^{-\int P\,dx} \)
  • B.\( e^{\int Q\,dx} \)
  • C.\( \int P\,dx \)
  • D.\( e^{\int P\,dx} \)

Answer: D

Multiplying by \( \mu = e^{\int P dx} \) makes the left side an exact derivative:

\[ \frac{d}{dx}\left(\mu y\right) \;=\; \mu\left(y' + Py\right) \;=\; \mu Q, \]

so \( \mu y = \int \mu Q\,dx \). The negative exponent would undo the grouping rather than create it, and the factor must be built from \( P \), the coefficient of \( y \), not from the right-hand side

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