#include<stdio.h>
#include<math.h>
void main()
{
    float a,b,c,x1,x2,disc;
    printf("please input a,b,c:");
    scanf("%d%d%d",&a,&b,&c);
    if(fabs(a)<1e-6)  //我把这个红色的1e-6唤作为“0”，为什么运行的结果就变了？
        printf("The equation is not a quadratic\n");
    else
    {
        disc=b*b-4*a*c;
        if(disc<0)
            printf("The equation has not real root\n");
        else
            if(fabs(disc)<0)
                printf("The equation has two equal roots:%8.4f\n",-b/(2*a));
            else
            {x1=(-b+sqrt(disc))/(2*a);
             x2=(-b-sqrt(disc))/(2*a);
             printf("The equation has distinct real roots:%8.4f,%8.4f\n",x1,x2);
            }
    }
}

我没看明白

你这个是求二元一次方程的吧

#include<stdio.h>
#include<math.h>
void main()
{
    int a,b,c,x1,x2,disc;
    printf("please input a,b,c:\n");
    scanf("%d%d%d",&a,&b,&c);
    disc=b*b-4*a*c;
    if(a==0)
        printf("The equation is not a quadratic\n");
    else
    {
        if(disc<0)
            printf("The equation has not real root\n");
        else
        {
            if(disc==0)
                printf("The equation has two equal roots:%5d\n",-b/(2*a));
            else
            {x1=(-b+sqrt(disc))/(2*a);
             x2=(-b-sqrt(disc))/(2*a);
             printf("The equation has distinct real roots:%5d,%5d\n",x1,x2);
            }
        }
    }
}

觉得假如是楼上所说，是求二次方程，我觉得肯能是逻辑错误，做了一些小小的改动，测试完好

严格说1e-6和0是不一样的，但是在计算机中有时候我建议用1e-6，这样得到的书比较准确，不然特殊情况下你可能得不到结果，尤其在工程上，你可以把0换成0.0，结果你就明白了？