треугольник вариант 1

#include <iostream>
#include <cmath>
#include <algorithm>
#include <iomanip>

using namespace std;
#define PI 3.14159265
struct otrezok
{
  double x1, y1, x2, y2, x, y;
  double length;
};

struct vershina
{
  double x, y;
};

vershina yravnenie(double a, double b, double c, double m)
{
  double x1=0, x2=0, D=0, x = 0, y = 0;

  if ((b==0) && (c==0))
      x = 1;
  else
     {
      D = b*b - 4*a*c;
             x1=(-b+sqrt(D))/2/a;
             x2=(-b-sqrt(D))/2/a;
              if (x1==x2)
                x = x1;
              else
                {
                  if (x1 < x2)
                   x = x2;
                  else x = x1;
                }
      }

  y = (m+b/2/m*x)/b*2*m;
  vershina C;
  C.x = x;
  C.y = y;
  return C;
}

int main ()
{
  otrezok AB, BC;
  double ygol_B;
  cin >> AB.length >> BC.length >> ygol_B;
  vershina B;

  B.x = 0;
  B.y = 0;

  AB.x = -sqrt(AB.length*AB.length/2);
  AB.y = -AB.x;

  double m = AB.length*BC.length*cos(ygol_B*PI/180.0);
  double AC_2 = AB.length*AB.length + BC.length*BC.length - 2*m;
  double a = 2*AB.x*AB.x;
  double b = -2*m*AB.x;
  double c = m*m - b*AB.y + 2*pow(AB.x, 4) - AC_2*a/2;
  vershina C = yravnenie(a, b, c, m);

  cout.precision(6);
  cout << fixed << B.x << " " << B.y << endl;
  cout << fixed << AB.x << " " << AB.y << endl;
  cout << fixed << C.x << " " << C.y << " ";
}