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Signals and systems electrical engineering miscellaneous

  1. If L[f(t)] =
    2(s + 2)
    , then ƒ(0+) and ƒ(∞) are given by
    s² + 2s + 5
    1. 0, 2 respectively
    2. 2, 0 respectively
    3. 0, 1 respectively
    4. 2/5, 0 respectively
Correct Option: B

2(s + 1)
=
2(s + 1)
= 2
s + 1
(s² + 2s + 5)(s² + 1)² + 5(s + 1)² + 4

This is standard Laplace Transform for the function
L| exp (– at) cos ωt | =
(s + a)
(s + a)² + ω²

where a = 1, ω = 2
Hence ƒ(t) = 2e– t cos 2t
ƒ(0) = ƒ(0+) = 2e– 0 cos 2 × 0 = 2
ƒ(∞) = 2e– ∞ t = 0



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