code: public static double getAngleInTriangle( double a, double b, double c ) {
          double angle = Math.acos((a * a + b * b - c * c) / (2.0 * a * b));
          return angle;
      }
docs: Uses the cosine rule to find an angle in radiants of a triangle defined by the length of its sides.
  
  <p>The calculated angle is the one between the two adjacent sides a and b.</p>
  
  @param a adjacent side 1 length.
  @param b adjacent side 2 length.
  @param c opposite side length.
  @return the angle in radiants.