用C求一元二次方程组的解，哪里出了错

#include<stdio.h>
#include<math.h>
int main(void)
{
float a,b,c;
float x1,x2,S;
printf("Please enter the coefficients a,b,c:\n");
scanf("%f,%f,%f",&a,&b,&c);
S=bb-4ac;
x1=(-b+sqrt(S))/2.0a;
x2=(-b-sqrt(S))/2.0*a;
if(S<0)
printf("error!\n");
else
printf("x1=%.4f, x2=%.4f\n",x1,x2);
return 0;

}

#include<stdio.h>
#include<math.h>
int main(void)
{
float a,b,c;
float x1,x2,S;
printf("Please enter the coefficients a,b,c:\n");
scanf("%f,%f,%f",&a,&b,&c);
S=b*b-4*a*c;
x1=(-b+sqrt(S))/2.0*a;
x2=(-b-sqrt(S))/2.0*a;
if(S<0)
printf("error!\n");
else
printf("x1=%.4f, x2=%.4f\n",x1,x2);
return 0;
}

如果是要除以2a的话

#include<stdio.h>
#include<math.h>
int main()
{
float a,b,c;
float x1,x2,S;
printf("Please enter the coefficients a,b,c:\n");
scanf("%f,%f,%f",&a,&b,&c);
S=b*b-4*a*c;
x1=(-b+sqrt(S))/(2.0*a);
x2=(-b-sqrt(S))/(2.0*a);
if(S<0)
printf("error!\n");
else
printf("x1=%.4f, x2=%.4f\n",x1,x2);
return 0;
}

我发现哪里错了，就是2.0*a没有加（）
但是乘的运算不是高于除吗，那不应该是加或不加（）都没关系吗？