表达式2: "y" equals "f" left parenthesis, "f" left parenthesis, "x" , right parenthesis , right parenthesisy=ffx
2
表达式3: "y" equals "f" left parenthesis, "f" left parenthesis, "f" left parenthesis, "x" , right parenthesis , right parenthesis , right parenthesisy=fffx