表达式2: "f" left parenthesis, "x" , right parenthesis equals Start sum from "n" equals 0 to "k" , end sum, StartFraction, left parenthesis, negative 1 , right parenthesis Superscript, "n" , Baseline "x" Superscript, 2 "n" , Baseline Over left parenthesis, 2 "n" , right parenthesis ! , EndFractionfx=k∑n=0−1nx2n2n!