What could be the problem with Run-Time Check Failure #3?

Question

C++ / C#

#include "pch.h"
#include <iostream>
#include <math.h>
using namespace std;
const double eps = 0.00001;
double _pow(double a, double b) {
  double c = 1;
  for (int i = 0; i < abs(b); i++) c *= a;
  if (b < 0) c = 1 / c;
  return c;
}
double _fact(double x) {
  if (x < 0) return 0; 
  if (x == 0) return 1; 
  else return x * _fact(x - 1);
}
double _sin(double x) {
  double a = x, s = x;
  int n2 = 1;
  while (fabs(a) > eps){
    a = ((-a)*x*x) / ((n2 + 1)*(n2 + 2));
    s += a;
    n2 += 2;
  }
  return s;
}
double _cos(double x) {
  double a=1.0, s=0.0;
  int p = (int)s;
  while (fabs(a) > eps) {
    s += a;
    p++;
    a = ((-a)*x*x) / ((2 * p - 1)*(2 * p));
  }
  return s;
}
double _tan(double x) {
  return (_sin(x) / _cos(x));
}
double proverka(double x1,double x2) {
  int b=0, c=0;
  if (x1 + x2 != -b && x1*x2 != c) cout << NULL << endl;
  else cout << "1" << endl;
  return x1,x2;
}
int main() {
  cout << "Vvedite zadachu kotoruyu nado proverit :" << endl;
  int h;
  cout << "2ZADACHA-1" << endl; cout << "1ZADACHA-2" << endl;
  cin >> h;
  switch (h) {
  case 1: {
    cout << "Enter A, B, C:\n";
    int a = 0, b = 0, c = 0;
    cin >> a >> b >> c;
    double x1, x2;
    int discriminant = 0;
    discriminant = ((int)_pow(b, 2)) - 4 * a * c;
    if (discriminant > 0 && a != 0) {
      x1 = ((-b - sqrt(discriminant)) / 2 * a);
      x2 = ((-b + sqrt(discriminant)) / 2 * a);
      cout << x1 << x2 << endl;
    }
    else if (discriminant = 0 && a != 0) cout << "" << endl;
    else cout << "korney net" << endl;
    proverka(x1, x2);
    break;
  }
  case 2: {
    double x, a, b;
    int swtch;
    cout << "Vvedite chislo dlya vibora" << endl;
    cout << "SIN-1" << endl; cout << "COS-2" << endl; cout << "POW-3" << endl; cout << "TAN-4" << endl; cout << "FACT-5" << endl;
    cin >> swtch;
    switch (swtch)
    {
    case 1: {
      cout << "Vvedite x:" << endl; cin >> x;
      cout << "sin " << x << " = " << _sin(x);
      break;
    }
    case 2: {
      cout << "Vvedite x:" << endl; cin >> x;
      cout << "cos " << x << " = " << _cos(x);
      break;
    }
    case 3: {
      cout << "Vvedite a:" << endl; cin >> a;
      cout << "Vvedite stepen:" << endl; cin >> b;
      cout << "pow " << a << " s stepenyu" << b << " = " << _pow(a, b);
      break;
    }
    case 4: {
      cout << "Vvedite x:" << endl; cin >> x;
      cout << "tan " << x << " = " << _tan(x);
      break;
    }
    case 5: {
      cout << "Vvedite x:" << endl; cin >> x;
      cout << "FACT! " << x << " = " << _fact(x);
      break;
    }
    default:break;
    }
  default:break; }
  }
}

Answer 1

It says "x2 was used without initialization"