/in/foo.c: In function 'main':
/in/foo.c:13:39: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
   13 |                         double x1=(-b+sqrt(delta))/(2*a);
      |                                       ^~~~
/in/foo.c:2:1: note: include '<math.h>' or provide a declaration of 'sqrt'
    1 | #include <stdio.h>
  +++ |+#include <math.h>
    2 | 
/in/foo.c:13:39: warning: incompatible implicit declaration of built-in function 'sqrt' [-Wbuiltin-declaration-mismatch]
   13 |                         double x1=(-b+sqrt(delta))/(2*a);
      |                                       ^~~~
/in/foo.c:13:39: note: include '<math.h>' or provide a declaration of 'sqrt'
/in/foo.c:22:16: error: 'o' undeclared (first use in this function)
   22 |         return o;
      |                ^
/in/foo.c:22:16: note: each undeclared identifier is reported only once for each function it appears in