二次方程的根

，所以我正在尝试编写可以计算二次方程的根的上图。这就是我想到的。第一个可能性(d> 0(有效，但其他可能性不做。我在代码中找不到错误。请你帮助我好吗？谢谢！

    #include <iostream>
    #include <cmath>
    using namespace std;
    int main(){
    float a;
    float b;
    float c;
    float x1;
    float x2;
    float D=b*b-4*a*c;
    float realPart;
    float imaginaryPart;
    cout << "Write a: " << endl;
    cin >> a;
    cout << "Write b: " << endl;
    cin >> b;
    cout << "Write c: " << endl;
    cin >> c;

   //first possibility D > 0
   if (D > 0){
      x1=(-b+sqrt(D))/(2*a);
      x2=(-b-sqrt(D))/(2*a);
      cout << "Results are: x1: " << x1 << " and x2 " << x2 << endl;
    }
   //second possibility D < 0
   else if (D < 0){
      realPart = -b/(2*a);
      imaginaryPart = (sqrt(-D))/(2*a);
      cout << "First result: " << realPart << " + " << imaginaryPart << "i" 
      <<endl;
      cout << "Second result: " << realPart << " - " << imaginaryPart << "i" 
      << endl;
    }
   else if (D == 0){
      x1=(-b)/(2*a);
      cout << "Result is: " << x1 << endl;
    }
   else
       cout << "THERE'S A MISTAKE!" << endl;
   return 0;
   }