ExplSyntaxOn
NewExpandableDocumentCommand{ForA}{ O {1} m m }{
fp_step_inline:nnnn { #2 } { #1 } { fp_eval:n { #2+#1*(#3-2) } } {  ##1, } fp_eval:n { #2+#1*(#3-1) } }
ExplSyntaxOff

ForA[1]{1}{9}

functiontableT{ 1/4,1/3,1/2,ForA[1]{1}{9},15}{ 1/#1^2 }

documentclass{article}
usepackage{xfp}
usepackage{xintexpr}
usepackage{siunitx}
sisetup{group-separator={,},output-decimal-marker={,}}

ExplSyntaxOn
seq_new:N l__ts_funcT_row_one_seq
seq_new:N l__ts_funcT_row_two_seq
NewDocumentCommand{functiontableT}{ m m }
 {% #1 = list of values, #2 = function
  group_begin:
   cs_set:Nn __ts_funcT_function:n  { #2 }
   seq_clear:N l__ts_funcT_row_one_seq
   seq_clear:N l__ts_funcT_row_two_seq
    clist_map_inline:nn { #1 }
      { seq_put_right:Nn l__ts_funcT_row_one_seq
             { $ xintFrac{xinttheexpr reduce( ##1 ) relax } $ }
        seq_put_right:Nx l__ts_funcT_row_two_seq
             { num{ fp_eval:n{round( __ts_funcT_function:n { fp_eval:n { ##1 } }, 2 ) } } }
       }
    use:x { exp_not:N begin{tabular}{ c *{seq_count:N l__ts_funcT_row_one_seq}{|c}} }
                            $ x $    & seq_use:Nn l__ts_funcT_row_one_seq { & }
                            tabularnewline hline
                            $ f(x) $ & seq_use:Nn l__ts_funcT_row_two_seq { & }
                        end{tabular}
  group_end:
 } %
ExplSyntaxOff

ExplSyntaxOn
NewExpandableDocumentCommand{ForA}{ O {1} m m }{
fp_step_inline:nnnn { #2 } { #1 } { fp_eval:n { #2+#1*(#3-2) } } {  ##1, } fp_eval:n { #2+#1*(#3-1) } }
ExplSyntaxOff

begin{document}

ForA[1]{1}{9}

functiontableT{ 1/4,1/3,1/2,1,2,3,4,5,6,7,8,9}{ 1/#1^2 }

end{document}