Expression 13: "f" Subscript, "n" , Baseline left parenthesis, "x" , right parenthesis equals "H" ( 1 minus "e" Superscript, StartFraction, negative "x" Over "S" , EndFraction , Baseline ) plus "I" plus StartFraction, "B" "x" Over "S" , EndFractionfnx=H(1−e−xS)+I+BxS