式3: "g" left parenthesis, "x" , right parenthesis equals Start sum from "k" equals 0 to "n" , end sum, StartFraction, "x" Superscript, left parenthesis, 2 "k" , right parenthesis , Baseline Over left parenthesis, 2 "k" , right parenthesis ! , EndFractiongx=n∑k=0x2k2k!