#include "stdio.h"
#include "math.h"
double area(double a,double b,double (*p)(double x));
double fun1(double x);
double fun2(double x);
double fun3(double x);
double fun4(double x);
main()
{
    double s;
    double a,b;
    int fun;
    scanf("%f%f",&a,&b);
    printf("the funtion is ");
    scanf("%d",&fun);
    switch(fun)
    {
        case 1:s=area(a,b,fun1); printf("%lf",s); break;
        case 2:s=area(a,b,fun2); printf("%lf",s);break;
        case 3:s=area(a,b,fun3); printf("%lf",s);break;
        case 4:s=area(a,b,fun4); printf("%lf",s); break;
    }
}
double area(double a,double b,double (*p)(double x))
{
    int i;
    double area,s=0,y1,y2,x,h;
    h=0.0001;
    x=a;
    y1=(*p)(x);
    for(i=1;a+i*h<b;i++) {
        x=a+i*h;
        y2=(*p)(x);
        area=(y1+y2)*h/2;
        y1=y2;
        s=s+area;}
    return s;
}
double fun1 (double x) {
    double f;
    f=x*x-2*x+2;
    return f;
}
double fun2 (double x) {
    double f;
    f=x*x*x+3*x*x-x+2;
    return f;
}
double fun3 (double x) {
    double f;
    f=x*sqrt(x+cos(2*x));
    return f;
}
double fun4 (double x) {
    double f;
    f=1/(1+x*x);
    return f;
}
上面的程序能够运行起来，没有编译器发现错误和警告，但是运行的结果是0.0000，不能显示正确的结果。求大侠解。