# include <stdio.h>
# include <math.h>

int main(void)
{
    double a, b, c, disc, x1, x2;
    printf("求解一元二次方程\n");
    printf("请输入a b c的值：\n");
    scanf("%lf %lf %lf", &a, &b, &c);
    disc = sqrt(b*b - 4*a*c);
    if (a<=0)//这里合适么？
        printf("这不是一元二次方程\n");
    else
    {
        if (sqrt<0)//这里合适么？
            printf("方程无实根\n");
        else if (sqrt>0)//同上
        {
            x1 = ((-b)+disc)/(2*a);
            x2 = ((-b)-disc)/(2*a);
            printf("方程两实根分别为：\nx1=%.2lf, x2=%.2lf\n", x1, x2);
        }
        else
        {
            x1 = ((-b)+disc)/(2*a);
            x2 = x1;
            printf("方程有一个实根：\nx1=x2=%.2lf\n", x1);
        }
    }

    return 0;
}

double delta = b*b-4*a*c;
if( a == 0 )
    非二次方程
else if( delta == 0 )
    两根相同
else if( delta > 0 )
    两实根
else // delta < 0
    两虚根

disc = sqrt(b*b - 4*a*c);
--- C标准如是说 A domain error occurs if the argument is less than zero.

if (sqrt<0)
--- 笔误了吧，应该是 if (disc<0)