式3: "y" minus "f" left parenthesis, "x" Subscript, 1 , Baseline , right parenthesis equals "g" left parenthesis, "x" Subscript, 1 , Baseline , right parenthesis left parenthesis, "x" minus "x" Subscript, 1 , Baseline , right parenthesisy−fx1=gx1x−x1