We show that PadB approximation can be effectively used for approximat
ion of performance functions in discrete-event systems, The method is
1) obtaining the MacLaurin coefficients of the performance function an
d 2) finding a PadB approximant from the MacLaurin coefficients and us
e it to approximate the function, We use the method with the expected
number of renewals in a random interval, GI/G/1 systems, and inventory
systems. The results are very good.