#include <stdio.h>
#define PI 3.14159
void strigon()
{
    float a,b,c;
    printf ("请输入三角形的底和高:\n");
    scanf ("%f %f",&a,&b);
    c=a*b/2;
    printf ("三角形的面积为:%f\n");
}
void circle()
{
   
    float r,s;
    printf ("请输入圆形的半径");
    scanf ("%f",&r);
    s=(PI*r*r)/2;
    printf ("圆的面积为%f",s);

}
void srectangle()
{
    float a,b,sum;
    printf ("请输入矩形的长:\n");
    scanf ("%f %f",&a,&b);
    sum=a*b;
    printf ("矩形的面积是:%f",sum);

}
void trapezoid()
{
    float a,b,h,sum;
    printf ("请输入梯形的上底、下底、高:\n");
    scanf  ("%f %f %f",a,b,h);
    sum=((a+b)*h)/2;
    printf ("梯形的面积为:%f",sum);

}
void main()
{
    int xz,tj=1;
    float a,b,h,s1;   
    printf("******************************************************\n\n");
    printf("          欢迎使用图形运算系统1.0by孙宇洪\n\n");   
    printf("******************************************************\n\n");
    printf("1.三角形面积的运算\n");   
    printf("2.圆形面积的运算\n");   
    printf("3.矩形面积的运算\n");   
    printf("4.梯形面积的运算\n");   
    printf("0.退出\n");
    while(tj)
    {
        printf("请选择图形运算：\n");
        scanf("%d",&xz);
        switch(xz)
        {
            case 1:
                strigon();break;
            case 2:
                circle();break;
            case 3:
                srectangle();break;
            case 4:
                trapezoid();break;
            case 0:   
            printf("欢迎再次使用，再见！\n");
            tj=0;break;
        }
        
               
     }
   
 

}

#include <stdio.h>
#define PI 3.14159
void strigon()
{
    float a,b,c;
    printf ("请输入三角形的底和高:\n");
    scanf ("%f %f",&a,&b);
    c=(a*b)/2;
    printf ("三角形的面积为:%f\n",c);
}
void circle()
{
   
    float r,s;
    printf ("请输入圆形的半径");
    scanf ("%f",&r);
    s=(PI*r*r)/2;
    printf ("圆的面积为%f",s);

}
void srectangle()
{
    float a,b,sum;
    printf ("请输入矩形的长:\n");
    scanf ("%f %f",&a,&b);
    sum=a*b;
    printf ("矩形的面积是:%f",sum);

}
void trapezoid()
{
    float a,b,h,sum;
    printf ("请输入梯形的上底、下底、高:\n");
    scanf  ("%f %f %f",&a,&b,&h);
    sum=((a+b)*h)/2;
    printf ("梯形的面积为:%f",sum);

}
void main()
{
    int xz,tj=1;
    float a,b,h,s1;   
    printf("******************************************************\n\n");
    printf("          欢迎使用图形运算系统1.0by孙宇洪\n\n");   
    printf("******************************************************\n\n");
    printf("1.三角形面积的运算\n");   
    printf("2.圆形面积的运算\n");   
    printf("3.矩形面积的运算\n");   
    printf("4.梯形面积的运算\n");   
    printf("0.退出\n");
    while(tj)
    {
        printf("请选择图形运算：\n");
        scanf("%d",&xz);
        switch(xz)
        {
            case 1:
                strigon();break;
            case 2:
                circle();break;
            case 3:
                srectangle();break;
            case 4:
                trapezoid();break;
            case 0:   
            printf("欢迎再次使用，再见！\n");
            tj=0;break;
        }
       
              
     }
    


} 

#include <stdio.h>
#include<stdlib.h>
#define PI 3.14159
void strigon()
{
    float a,b,c;
    printf ("请输入三角形的底和高:\n");
    scanf ("%f%f",&a,&b);
    c=a*b/2;
    printf ("三角形的面积为:%f\n",c);
}
void circle()
{
   
    float r,s;
    printf ("请输入圆形的半径:");
    scanf ("%f",&r);
    s=(PI*r*r);
    printf ("圆的面积为%f\n",s);
}
void srectangle()
{
    float a,b,sum;
    printf ("请输入矩形的长:\n");
    scanf ("%f %f",&a,&b);
    sum=a*b;
    printf ("矩形的面积是:%f\n",sum);

}
void trapezoid()
{
    float a,b,h,sum;
    printf ("请输入梯形的上底、下底、高:\n");
    scanf  ("%f %f %f",&a,&b,&h);
    sum=((a+b)*h)/2;
    printf ("梯形的面积为:%f\n",sum);

}
void main()
{
    int xz,tj=1;
    while(tj)
    {
        system("cls");
        printf("\t\t******************************************************\n\n");
        printf("\t\t**          欢迎使用图形运算系统1.0by孙宇洪        ** \n\n");   
        printf("\t\t******************************************************\n\n");
        printf("\t\t\t1.三角形面积的运算\n");   
        printf("\t\t\t2.圆形面积的运算\n");   
        printf("\t\t\t3.矩形面积的运算\n");   
        printf("\t\t\t4.梯形面积的运算\n");   
        printf("\t\t\t0.退出\n");
        printf("\t\t******************************************************\n\n");
  
        printf("\t\t\t请选择图形运算:");
        scanf("%d",&xz);
        switch(xz)
        {
            case 1:
                strigon();system("pause");break;
            case 2:
                circle();system("pause");break;
            case 3:
                srectangle();system("pause");break;
            case 4:
                trapezoid();system("pause");break;
            case 0:   
            printf("欢迎再次使用，再见！\n");
            tj=0;break;
        }
      
               
     }
   


}

帮你改了错误~~美化了下这个系统：

#include <stdio.h>
#define PI 3.14159
void strigon()
{
    float a,b,c;
    printf ("请输入三角形的底和高:\n");
    scanf ("%f %f",&a,&b);
    c=0.5*a*b;
    printf ("三角形的面积为:%f\n",c);
}
void circle()
{
   
    float r,s;
    printf ("请输入圆形的半径");
    scanf ("%f",&r);
    s=PI*r*r;
    printf ("圆的面积为%f",s);

}
void srectangle()
{
    float a,b,sum;
    printf ("请输入矩形的长:\n");
    scanf ("%f %f",&a,&b);
    sum=a*b;
    printf ("矩形的面积是:%f",sum);

}
void trapezoid()
{
    float a,b,h,sum;
    printf ("请输入梯形的上底、下底、高:\n");
    scanf  ("%f %f %f",a,b,h);
    sum=((a+b)*h)/2;
    printf ("梯形的面积为:%f",sum);

}
void main()
{
    int xz,tj=1;
   
    printf("******************************************************\n\n");
    printf("          欢迎使用图形运算系统1.0by孙宇洪\n\n");   
    printf("******************************************************\n\n");
    printf("1.三角形面积的运算\n");   
    printf("2.圆形面积的运算\n");   
    printf("3.矩形面积的运算\n");   
    printf("4.梯形面积的运算\n");   
    printf("0.退出\n");
    while(tj)
    {
        printf("请选择图形运算：\n");
        scanf("%d",&xz);
        switch(xz)
        {
            case 1:
                strigon();break;
            case 2:
                circle();break;
            case 3:
                srectangle();break;
            case 4:
                trapezoid();break;
            case 0:   
            printf("欢迎再次使用，再见！\n");
            tj=0;break;
        }
        
               
     }
   


}