#include<stdio.h>
#include<math.h>
float f(float x)           /*定义f函数，以实现f(x)=x^3-5*x^2+16*x-80*/
{
    float y;
    y=((x-0.5)*x+16.0)*x-80.0;
    return(y);
}
float xpoint(float x1,float x2)      /*定义xpoint函数，求出弦与x轴的交点*/
{
    float y;
    y=(x1*f(x2)-x2*f(x1))/(f(x2)-f(x1));
    return(y);
}
float root(float x1,float x2)    /*定义root函数，求近似根*/
{
    float x,y,y1;
    y1=f(x1);
    do
    {
        x=xpoint(x1,x2);
        y=f(x);
        if(y*y1>0)      /*f(x)与f(x1)同符号*/
        {
            y1=y;
            x1=x;
        }
        else
            x2=x;
        while(fabs(y)>=0.0001);
        return(x);

}
    void main()        /*主函数*/
    {
        float x1,x2,f1,f2,x;
        do
        {
            printf("input x1,x2:\n");
            scanf("%f,%f",&x1,&x2);
            f1=f(x1);
            f2=f(x2);

        }
        while(f1*f2>=0);
            x=root(x1,x2);
        printf("a root of equation is %8.4f\n",x);
    }