二楼代码-如有计算错误-请指出

#include<iostream>
#include<cmath>
using namespace std;
int main()
{
double a,b,c,d,x1,x2;
S:
cout<<"请输入系数a,b,c："<<endl;
cin>>a>>b>>c;
d=b*b-4*a*c;
if(a==0)
{
x1=-c/b;
cout<<a<<"x^2+"<<b<<"x+"<<c<<"=0"<<endl;
cout<<"这是一元一次方程！\nx="<<x1<<endl;
} else {
if (d>0)
{
x1=(-b+sqrt(d))/2*a;
x2=(-b-sqrt(d))/2*a;
cout<<a<<"x^2+"<<b<<"x+"<<c<<"=0"<<endl;
cout<<"x1="<<x1<<endl;
cout<<"x2="<<x2<<endl;
}
else if (d==0)
{
x1=-b/2*a;
cout<<a<<"x^2+"<<b<<"x+"<<c<<"=0"<<endl;
cout<<"x1=x2="<<x1<<endl;
}
else if (d<0)
{
//x1=(-b+sqrt(-d))/2*a;
//x2=(-b-sqrt(-d))/2*a;
cout<<a<<"x^2+"<<b<<"x+"<<c<<"=0"<<endl;
cout<<" "<<-b<<"+i"<<sqrt(-d)<<endl;
cout<<"x1 = ——————————————"<<endl;
cout<<" "<<2*a<<endl;
cout<<" "<<-b<<"-i"<<sqrt(-d)<<endl;
cout<<"x2 = ——————————————"<<endl;
cout<<" "<<2*a<<endl;
}
}
goto S;
return 0;
}