We assume you want to find the inverse transform of s/(s^2 +3s -4). This can be written in partial fraction form as
(4/5)/(s+4) + (1/5)/(s-1)
which can be found in a table of transforms to be the transform of
(4/5)e^(-4t) + (1/5)e^t
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There are a number of ways to determine the partial fractions. They all start with factoring the denominator.
s^2 +3x -4 = (s+4)(s-1)
After that, you can postulate the final form and determine the values of the coefficients that make it so. For example:
A/(s+4) + B/(s-1) = ((A+B)s + (4B-A))/(s^2 +3x -4)
This gives rise to two equations:
(A+B) = 1
(4B-A) = 0
