#include<stdio.h>
#include<math.h>
float DaYu(int a,int b,int c);
float XiaoYu(int a,int b,int c);
float DengYu(int a,int b,int c);
int main()
{
    int m,n,l;
    float x1,x2,t,result;
    printf("qing shu ru m(m!=0),n,l:\n");
    scanf("%d,%d,%d",&m,&n,&l);
    t=n*n-4*m*n;
    if(t>0)
    {
        result=DaYu(m,n,l);
        printf("%.2f,%.2f",x1,x2);
    }
    else if(t<0)
    {
        result=XiaoYu(m,n,l);
        printf("wujie");
    }
    else
    {
        result=DengYu(m,n,l);
        printf("%.2f",x1);
   
    return 0;

}
float DaYu(int a,int b,int c)
{
    float x1,x2,t;
    x1=(-b+sqrt(t))/(2*a);
    x2=(-b-sqrt(t))/(2*a);
    return x1,x2;
}

float XiaoYu(int a,int b,int c)
{
    printf("wujie");
}
float DengYu(int a,int b,int c)
{
    float x1;
    x1=-b/2a;
}
错误：
test37.cpp
F:\c 源代码\test37\test37.cpp(12) : warning C4244: '=' : conversion from 'int' to 'float', possible loss of data
F:\c 源代码\test37\test37.cpp(32) : error C2601: 'DaYu' : local function definitions are illegal
F:\c 源代码\test37\test37.cpp(40) : error C2601: 'XiaoYu' : local function definitions are illegal
F:\c 源代码\test37\test37.cpp(44) : error C2601: 'DengYu' : local function definitions are illegal
F:\c 源代码\test37\test37.cpp(46) : error C2059: syntax error : 'bad suffix on number'
F:\c 源代码\test37\test37.cpp(48) : fatal error C1004: unexpected end of file found

这个代码是行不通的。我就帮你把编译的错误改了，不过结果还是错的，方法的问题
#include<stdio.h>
#include<math.h>
float DaYu(float a,float b,float c);
float XiaoYu(float a,float b,float c);
float DengYu(float a,float b,float c);
int main()
{
    int m,n,l;
    float x1,x2,t,result;
    printf("qing shu ru m(m!=0),n,l:\n");
    scanf("%d,%d,%d",&m,&n,&l);
    t=n*n-4*m*l;
    if(t>0)
    {
        result=DaYu(m,n,l);
        printf("%.2f,%.2f",x1,x2);
    }
    else if(t<0)
    {
        result=XiaoYu(m,n,l);
    }
    else
    {
        result=DengYu(m,n,l);
        printf("%.2f",x1);
    }
   
    return 0;

}
float DaYu(float a,float b,float c)
{
    float x1,x2,t;
    x1=(-b+sqrt(t))/(2*a);
    x2=(-b-sqrt(t))/(2*a);
    return x1,x2;
}

float XiaoYu(float a,float b,float c)
{
    printf("wujie");
}
float DengYu(float a,float b,float c)
{
    float x1;
    x1=-b/2*a;
}

自己单步跟跟看

为了分数，给出代码：

#include<stdio.h>
#include<math.h>
int main(void)
{
    int a,b,c;
    double t;
    printf("qing shu ru a(a!=0),b,c:");
    scanf("%d,%d,%d",&a,&b,&c);
    t = b * b - 4.0 * a* c;
    if ( t > 0 )
    {
        printf( "x1 = %.2f,x2 = %.2f\n", (0-b+sqrt(t))/2.0*a, (0-b-sqrt(t))/2.0*a );
    }
    else if( t < 0 )
    {
        printf( "wujie\n" );
    }
    else
    {
        printf( "x1 = x2 = %.2f\n", (0-b)/2.0*a );
    }
    return 0;
}

另外，给个最真诚的建议，回去好好看基础书，入门书……
下次再写出那样的代码就有找骂的嫌疑了。