Fusion code

#! /bin/bash

files=(*.txt)

for ((i=0; i<${#files[@]}; i++)) ; do
    file="${files[$i]}"
    awk -f f.awk "$file" > "${file}.mod"
done

 f.awk is:
{
    gsub(/\([^)]*\)/,"&; printf&")
    print
}

input.txt:

x=7;
sqrt(x+5*4); sqrt(x*x); 
i=3;
a=2+sqrt(8);

Running: awk -f f.awk input.txt gives:
x=7;
sqrt(x+5*4); printf(x+5*4); sqrt(x*x); printf(x*x); 
i=3;
a=2+sqrt(8); printf(8);

This wont work for nested parenthesis, like sqrt(4+2*(x+y))
Running: awk -f f.awk input.txt>>output.txt writes it to output file