/in/foo.c: In function 'main':
/in/foo.c:22:15: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
    x1 = (-b + sqrt(d)) / (2 * a);
               ^~~~
/in/foo.c:22:15: warning: incompatible implicit declaration of built-in function 'sqrt'
/in/foo.c:22:15: note: include '<math.h>' or provide a declaration of 'sqrt'
/in/foo.c:30:8: warning: incompatible implicit declaration of built-in function 'sqrt'
    i = sqrt(-d) / (2 * a);
        ^~~~
/in/foo.c:30:8: note: include '<math.h>' or provide a declaration of 'sqrt'