Find the inverse Laplace transform of the following: F(s)=(2s+1)/(s^2−2s+2)

trumansoftjf0

trumansoftjf0

Answered question

2022-11-11

Find the inverse Laplace transform of the following:
F ( s ) = 2 s + 1 s 2 2 s + 2 .
The answer is f ( t ) = 2 e t cos t + 3 e t sin t
Obviously once you have the decomposed fraction the remainder of the problem is simple but I can't seem to get to that point. Could someone please lay out the steps to decompose F(s)?

Answer & Explanation

Kayleigh Cross

Kayleigh Cross

Beginner2022-11-12Added 19 answers

You want to set things up to use the formulas
L [ e a t sin ( k t ) ] = k ( s a ) 2 + k 2 , L [ e a t cos ( k t ) ] = s a ( s a ) 2 + k 2
Towards this end, write
2 s + 1 s 2 2 s + 2 = 2 s + 1 ( s 1 ) 2 + 1 = 2 s 2 + 2 + 1 ( s 1 ) 2 + 1 = 2 ( s 1 ) ( s 1 ) 2 + 1 + 3 ( s 1 ) 2 + 1 .
Note that in the third equality above, we had to do something somewhat sneaky in order to use our formulas. We needed a term of the form a ( s 1 ) or a constant term upstairs (just 2 s + 1 won't do). Writing 2 s + 1 = 2 ( s 1 ) + 3 allowed us to appeal to the formulas.
The salient thing to note in this problem is that the denominator of the original expression does not factor into linear terms. So, complete the square to obtain ( s 1 ) 2 + 1; and then the formulas above should spring to mind.

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