The following is a part of a currentI(s)(in $\displaystyle s$-space):

What am I doing wrong in using this formula for finding $\displaystyle A$ and $\displaystyle B$:

; where $\displaystyle a$ is a pole.

PROBLEM

I easily findA, but notB:

end of PROBLEM

The solution (which is correct) is

What am I doing wrong inside the PROBLEM section? Or is the formula for $\displaystyle A$ (or $\displaystyle B$) wrong?