if (a==0)
{
if ((b==0)&&(c==0))
cout<<"Phuong trinh co vo so nghiem.\n";
else if((b==0)&&(c!=0))
cout<<"Phuong trinh vo nghiem.\n";
else
cout<<"Phuong trinh co 1 nghiem x = "<<-c/b;
}

else {
float delta=b*b-4*a*c;

if (delta<0)
{cout<<"Phuong trinh vo nghiem.\n";}
else {
if (delta==0)
{cout<<"Phuong trinh co nghiem kep x1 = x2 = "<<-b/(2*a)<<"\n";}
else
{cout<<"Phuong trinh co 2 nghiem phan biet.\n";
cout<<"Nghiem x1 = "<<(-b-sqrt(delta))/2*a<<"\n";
cout<<"Nghiem x2 = "<<(-b+sqrt(delta))/2*a<<"\n";}
}
}

system("pause");

}

cách 2:

#include<iostream>
#include<math.h>
using namespace std;
int main()
{
int a,b,c;
cout<<"nhap vao cac so "<<endl;
cout<<"a= ";cin>>a;
cout<<"b= ";cin>>b;
cout<<"c= ";cin>>c;
if (a==0)
{
if(b==0&&c==0)
cout<<" phuong trinh vo so nghiem"<<endl;
if(b==0&&c!=0)
cout<<"phuong trinh vo nghiem"<<endl;
if(b!=0&&c!=0)
cout<<"phuong trinh co 1 nghiem X= "<<-c/b<<endl;
}
if (a!=0)
{
float del = b*b-4*a*c;
if (del<0)
cout<<"phuong trinh vo nghiem"<<endl;
if (del==0)
cout<<"phuong trinh co nghiem kep X= "<<-b/(2*a)<<endl;
if(del>0)
{
cout<<"phuong trinh co 2 nghiem phan biet X1,X2"<<endl;
cout<<"X1= "<<(-b + sqrt(del))/(2*a)<<endl;
cout<<"X2= "<<(-b - sqrt(del))/(2*a)<<endl;
}
}
system("pause");
}