Expression 4: "y" equals 1 minus StartFraction, left parenthesis, StartNestedFraction, "a" ! NestedOver left parenthesis, "a" minus "x" , right parenthesis ! , EndNestedFraction , right parenthesis Over "a" Superscript, "x" , Baseline , EndFraction left brace, 1 less than or equal to "x" less than "a" plus 1 , right bracey=1−a!a−x!ax1≤x<a+1