I think it would be easiest to use an array of chars. Say this for example (if it works)
Var
myEquation : array[1..256] of char;
Equ : string[255];
my_file :text;
BEGIN
assign(my_file, 'input.txt');
reset(my_file);
read(my_file, Equ);
myEquation := Equ
Writeln(myEquation[1..10])
not sure if this helps at all, but that would atleast put my equation into the array and display the equation (had the writeln to make sure the equation was in the array)