Expression 2: "y" equals Start sum from "n" equals 0 to "a" , end sum, StartFraction, left parenthesis, negative 1 , right parenthesis Superscript, "n" , Baseline "x" Superscript, left parenthesis, 2 "n" plus 1 , right parenthesis , Baseline Over left parenthesis, 2 "n" plus 1 , right parenthesis ! , EndFractiony=a∑n=0−1nx2n+12n+1!