/in/foo.c: In function 'main':
/in/foo.c:5:6: error: 'i' undeclared (first use in this function)
    5 |  for(i=1;i<=n;i++)
      |      ^
/in/foo.c:5:6: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:6:16: warning: implicit declaration of function 'pow' [-Wimplicit-function-declaration]
    6 |          Sn=Sn+pow(a,i);
      |                ^~~
/in/foo.c:2:1: note: include '<math.h>' or provide a declaration of 'pow'
    1 | #include <stdio.h>
  +++ |+#include <math.h>
    2 | int main()
/in/foo.c:6:16: warning: incompatible implicit declaration of built-in function 'pow' [-Wbuiltin-declaration-mismatch]
    6 |          Sn=Sn+pow(a,i);
      |                ^~~
/in/foo.c:6:16: note: include '<math.h>' or provide a declaration of 'pow'