Expression 2: "y" Subscript, 1 , Baseline tilde "a" times "e" Superscript, negative "b" times "x" Subscript, 1 , Baseline , Baseline times cosine left parenthesis, "k" left parenthesis, "x" Subscript, 1 , Baseline minus "d" , right parenthesis , right parenthesis plus "c"y1~a·e−b·x1·coskx1−d+c