#include <stdio.h>
#include <math.h>
float f (float x)
{
    float y;
    y = x * x * x + 5 * x * x + 16 * x - 80;
    return y;
}
float xpoint (float x1, float x2)
{
    float x;
    x = (x1 * f (x2) - x2 * f (x1)) / (f(x2) - f (x1));
    return x;
}
float root (float x1, float x2)
{
    float x, y, y1;
    y1 = f (x1);
    do
    {
        x = xpoint (x1, x2);
        y = f (x);
        if (y * y1 > 0)
        {
            y1 = y;
            x1 = x;
        }
        else
        {
            x2 = x;
        }
    }
    while (fabs (y) > 0.00001);
    return x;
}
void main ()
{
    float x, x1, x2, y1, y2;
    do
    {
        printf ("please enter x1 and x2:");
        scanf ("%f%f", &x1, &x2);
        y1 = f (x1);
        y2 = f (x2);
    }
    while (y1 * y2 > 0);
    x = root (x1, x2);
    printf ("a root is:%f\n", x);
}

    do
    {
        printf ("please enter x1 and x2:");
        scanf ("%f%f", &x1, &x2);
        y1 = f (x1);
        y2 = f (x2);
    }
    while (y1 * y2 > 0);


如果while值为真，要一直输下去中吗？

//输入x1,x2一直到对应的y1,y2乘积小于0为止，计算此时root（x1,x2）
//例如你输入2,3就有结果出来了！
//程序没问题

是，直到y1与y2的符号相反为止