#include<stdio.h>
#include<math.h>
float x1,x2;
double p,q,di;

greater_than_zero(float a,float b);                  
{
    x1=(-b+sqrt(di))/(2*a);
    x2=(-b-sqrt(di))/(2*a);
}
equal_to_zero(float a,float b);
            
{
    x1=x2=-b/(2*a);
}
smaller_than_zero(float a, float b);
                  
{
    p=-b/(2*a);
    q=sqrt(-di)/(2*a);
}
 main()
{
    float a,b,c;
    printf("请输入方程的各项系数");
    scanf("%f,%f,%f",&a,&b,&c);
   
    printf("方程%6.3fx*x+%6.3fx+%6.3f=0的解为:\n",a,b,c);
    di=b*b-4*a*c;
   
    if(di>0)
    {
        greater_than_zero(a,b);
        printf("x1=%6.3f,x2=%6.3f\n",x1,x2);
    }
    if(di=0)
    {
        equal_to_zero(a,b);
        printf("x1=x2=%6.3f\n",x1);
    }D:\新建文件夹\Microsoft Visual Studio\MyProjects\求根程序\求根.cpp(7) : error C2447: missing function header (old-style formal list?)
D:\新建文件夹\Microsoft Visual Studio\MyProjects\求根程序\求根.cpp(13) : error C2447: missing function header (old-style formal list?)
D:\新建文件夹\Microsoft Visual Studio\MyProjects\求根程序\求根.cpp(18) : error C2447: missing function header (old-style formal list?)

greater_than_zero(float a,float b); 把“；”去掉，
float转换为double可以，
double转换为float要强制转换,
你再试试吧
                 

#include<stdio.h>
 #include<math.h>
 double x1,x2;
 double p,q,di;
 void greater_than_zero(float a,float b,float c)                  
 {
     x1=(-b+sqrt(di))/(2*a);
     x2=(-b-sqrt(di))/(2*a);
     printf("x1=%6.3lf,x2=%6.3lf\n",x1,x2);
 }
void equal_to_zero(float a,float b,float c)
              
{
     x1=x2=-b/(2*a);
     printf("x1=x2=%6.3f\n",x1);
 }
void smaller_than_zero(float a, float b,float c)
                  
 {
     p=-b/(2*a);
     q=sqrt(fabs(di))/(2*a);
     printf("p=%f,q=%f",p,q);
 }
void main()
 {
     float a,b,c;
     printf("请输入方程的各项系数");
     scanf("%f,%f,%f",&a,&b,&c);
     printf("方程%6.3fx*x+%6.3fx+%6.3f=0的解为:\n",a,b,c);
     di=b*b-4*a*c;
    if(di>0)
     {
         greater_than_zero(a,b,c);
     }
     else if(di=0)
     {
         equal_to_zero(a,b,c);
     }
     else
     {
         smaller_than_zero(a,b,c);
     }
        
}