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Laplace transform of \(\delta(t - T)\) is :

  • A.constant
  • B.\(1/e^{-sT}\)
  • C.\(e^{-sT}\)
  • D.\(s\,e^{-sT}\)

Answer: C

The Laplace transform of \(\delta(t - T)\) is \(e^{-sT}\).

Using the sifting property of the impulse, for \(T \gt 0\):

\[ \int_{0}^{\infty} \delta(t - T) e^{-st}\, dt = e^{-sT} \]

This is also the time-shift theorem applied to \(\mathcal{L}\{\delta(t)\} = 1\): delaying by T multiplies the transform by \(e^{-sT}\).

'Constant' is the transform of the undelayed impulse \(\delta(t)\). \(1/e^{-sT} = e^{sT}\) would be an advance, not a delay, and \(s e^{-sT}\) is the transform of the derivative of a delayed impulse.

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