law of cosine(s)



  angleA = Math.acos((b*b + c*c - a*a) / (2 * b * c));
  angleB = Math.acos((a*a + c*c - b*b) / (2 * a * c));
  angleC = Math.acos((a*a + b*b - c*c) / (2 * a * b));