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The final value of \(2S + 1 / S^{4} + 8S^{3} + 16S^{2} + S\) is

  • A.0
  • B.\(\infty\)
  • C.1
  • D.2

Answer: C

The final value is 1, option (C). By the final value theorem, \(\lim_{t\to\infty} f(t) = \lim_{s\to 0} sF(s)\), provided that all the poles of \(sF(s)\) lie in the left half plane.

\[ sF(s) = \frac{s(2s+1)}{s\,(s^{3}+8s^{2}+16s+1)} \]

\[ = \frac{2s+1}{s^{3}+8s^{2}+16s+1} \]

\[ \lim_{s\to0} sF(s) = \frac{1}{1} = 1 \]

The cubic in the denominator is stable, since all its coefficients are positive and \(8\times16 > 1\), so the theorem applies. The answers 0, \(\infty\) and 2 in options (A), (B) and (D) do not follow from this limit.

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