Code tính tổ hợp trong C#

public double GiaiThua(int n)
{
   double giaithua = 1;
   for (int i = 2; i <= n; i++)
      giaithua *= i;
   return giaithua;
}

public double ToHop(int k, int n)
{
   return GiaiThua(n) / (GiaiThua(k) * GiaiThua(n - k));
}

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