Find sum of numbers C lang

I need to calculate the sum of the number a (double) to the power of n (int). Without pow function !!!!!
I will be very grateful.

I have done this way, but it is very complex, I want something simpler.

If you know how to make this via Java or C++ or Pascal please reply too)

1/a2 + 1/a4 + 1/a6 … + 1/a2n.

 
var i, n: integer;
    s, a, x: real;
    f: boolean;
 
begin
  repeat
    write('n = '); readln(n);
    if n < 1 then writeln('Error: n <= 0, reenter.')
  until n >= 1;
  repeat
    write('a = '); readln(a);
    if a = 0 then writeln('Error: a = 0, reenter.')
  until a <> 0;
  s := 0;
  x := 1;
  f := true;
  for i := 1 to n do
    begin
      x := x / a / a;
      if x = 0
        then begin
          writeln('Float rounding error.');
          f := false;
          break
        end;
      s := s + x
    end;
  if f then writeln('s = ', s);
  readln
end.```

>Solution :

In C:

double bPow = 1;
double sum = 0
double b = a * a;

for (int i = 0; i < n; i++)
{
    bPow *= b;
    sum += 1 / bPow;
}

This gives you the sum of all 1/a^x for x = [2, 4, …, 2n] .