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The integrating factor of the differential equation \((1 + x^{2})dy = (1 - 2xy)dx\) is

  • A.\(x^{2} + 1\)
  • B.\(\log_{e}(x^{2} + 1)\)
  • C.\(\frac{x}{x^{2} + 1}\)
  • D.\(x(x^{2} + 1)\)
  • E.\(\log_{e}|x|\)

Answer: A

The integrating factor is \(x^{2} + 1\).

Put the equation in linear form:

\[ \frac{dy}{dx} + \frac{2x}{1 + x^{2}}y = \frac{1}{1 + x^{2}} \]

The integrating factor is \(e^{\int P\,dx} = e^{\log(1 + x^{2})} = 1 + x^{2}\). \(\log_{e}(x^{2} + 1)\) stops before taking the exponential, which is the usual slip; the other options do not come from \(\int\frac{2x}{1 + x^{2}}dx\).

Multiplying through by \(1 + x^{2}\) gives \(\frac{d}{dx}[(1 + x^{2})y] = 1\), so the solution is \((1 + x^{2})y = x + C\).

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