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For complex roots \( \alpha \pm i\beta \), the complementary function is

  • A.\( C_{1}\cos\beta x + C_{2}\sin\beta x \) with no exponential
  • B.\( \left(C_{1} + C_{2}x\right)e^{\alpha x} \)
  • C.\( e^{\alpha x}\left(C_{1}\cos\beta x + C_{2}\sin\beta x\right) \)
  • D.\( e^{\beta x}\left(C_{1}\cos\alpha x + C_{2}\sin\alpha x\right) \)

Answer: C

The pair \( e^{(\alpha \pm i\beta)x} \) combines through Euler's formula into a real exponential envelope times an oscillation:

\[ y \;=\; e^{\alpha x}\left(C_{1}\cos\beta x + C_{2}\sin\beta x\right). \]

The real part sets the growth or decay and the imaginary part the frequency, so interchanging them would exchange the envelope for the oscillation

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