这是我写的一个求一元二次根的程序，但当b*b-4ac<0时的根求不出来，请帮我看看错在哪里？谢谢！
#include "stdio.h"
#include "math.h"
double x1,x2,disc,m,n;

greater_zero(double a,double b)
{
    
    x1=((-b+sqrt(disc))/(2*a));
    x2=((-b-sqrt(disc))/(2*a));
}

equa_zero(double a,double b)
{
    
    x1=-b/(2*a);
}

smaller_zero(double a,double b)
{
    double m,n;
    m=-b/(2*a);
    n=sqrt(disc)/(2*a);
}

main()
{double a,b,c;
    scanf("%lf%lf%lf",&a,&b,&c);
    printf("方程为：%.3lfx*x+%.3lfx+%.3lf=0\n",a,b,c);
    disc=b*b-4*a*c;
    printf("方程的解是：\n");
    if(disc>0)
      {
          greater_zero(a,b);
          printf("x1=%lf\nx2=%lf",x1,x2);
          }
      else if(disc==0)
      {
          equa_zero(a,b);
          printf("x1=x2=%lf",x1);
      }
      else
      {
          smaller_zero(a,b);
          printf("x1=%lf+%lfi\nx2=%lf-%lfi\n",m,n,m,n);
          printf("%lf",m);
          }
}