/in/foo.c: In function 'main':
/in/foo.c:20:44: error: 'i' undeclared (first use in this function)
   20 |                         x1=(-b-sqrt(delta)*i)/(2*a);
      |                                            ^
/in/foo.c:20:44: note: each undeclared identifier is reported only once for each function it appears in