calculating angle between two lines

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I am working on a research project to detect and track hand, then move objects according to hand's motion. I need to calculate the angle between a horizontal axis and the centroid of hand, and update the angle every time the hand move. I implemented code, but it doesn't calculate the angle properly. The output angle has a repeated values i.e (114, -114, 57, 0, -57) and the single value is repeated for several frames, even if i moved my hand.

Here is the code :-

                    if (dArea > 10000)
            {
        //calculate the position of hand
        int posX = dM10 / dArea;
        int posY = dM01 / dArea;
        if (iLastX >= 0 && iLastY >= 0 && posX >= 0 && posY >= 0)
        {
            line(imgLines, Point(200, 200), Point(400, 200), Scalar(255), 2, 8, 0);
            line(imgLines, Point(200, 200), Point(posX, posY), Scalar(0, 0, 255), 2, 8, 0);

        }
        int x1 = 400;
        int y1 = 200;
        int x2 = posX;
        int y2 = posY;
        int X = x2 - x1;
        int Y = y2 - y1;
        int x = pow((double)X, 2); //type casting from int to double
        int y = pow((double)Y, 2);
        int d = abs(sqrt((double)x + (double)y));
        int angleInRadian = atan2(Y,X); //angle in radian
        int angleInDegree = angleInRadian * 180 / M_PI; //angle in degree
        if (angleInDegree < 0) {
            angleInDegree += 2 * M_PI;
        }
        string Result;          // string which will contain the result

        ostringstream convert;   // stream used for the conversion

        convert << angleInDegree;      // insert the textual representation of 'Number' in the characters in the stream

        Result = convert.str();
        putText(imgOriginal, Result, Point2f(iLastX, iLastY), FONT_HERSHEY_PLAIN, 2, Scalar(0, 0, 255, 255));
        iLastX = posX;
        iLastY = posY;
    }

• How to calculate the angle properly?
• how to keep only the last line and erase the previous ones?