Tracer un arc

/**
 * @return x, solution of (a+bx)²+(c+dx)²=e²
 */
private static double resolve(double a, double b, double c, double d, double e) {
    // http://www.wolframalpha.com/input/?t=crmtb01&i=(a%2Bb*x)%C2%B2%2B(c%2Bd*x)%C2%B2%3De%C2%B2
    return (-Math.sqrt(2*a*b*c*d - a*a*d*d - b*b*c*c + b*b*e*e + d*d*e*e) - (a*b+c*d)) / (b*b+d*d);
}

public static void main(String[] args) {

    // Parameters
    //------------
    
    double x1 = 300, y1 = 100;
    double x2 = 700, y2 = 300;
    double h = 100; // >0 and <=p
    
    // Computation
    //------------
    
    double x3 = (x1+x2)/2, y3 = (y1+y2)/2; // (x3,y3) middle of (x1,y1) and (x2,y2)
    double p = Math.hypot(x3-x1,y3-y1); // distance between (x1,y1) and (x3,y3) solve (x3-x1)²+(y3-y1)²=p²
    double r = (p*p+h*h)/(2*h); // on right triangle, solve p²+(r-h)²=r²
    
    double a = (y1-y2) / (x1-x2), b = (x1*y2 - x2*y1) / (x1-x2); // y=ax+b for (x1,y1) and (x2,y2)
    double c = -1/a, d = y3-c*x3; // line y=cx+d is perpendicular with line y=ax+b
    
    // y0=c.x0+d so distance between (x0,y0) and (x3,y3) solve (x3-x0)²+(y3-(c.x0+d))² = (r-h)²
    double x0 = resolve(x3, -1, y3-d, -c, r-h), y0 = c*x0+d;
    
    double alpha1 = Math.atan2(y1-y0,x1-x0), alpha2 = Math.atan2(y2-y0,x2-x0);
    
    // Display
    //------------
    
    int x = (int) Math.round(x0 - r), y = (int) Math.round(y0 - r), width = (int) Math.round(2*r), height = width;
    // angles are negative because Swing origin is on top-left corner with descending ordinates
    int startAngle = (int) -Math.round(Math.toDegrees(alpha1)), arcAngle = (int) -Math.round(Math.toDegrees(alpha2 - alpha1));
    
    JFrame frame = new JFrame();
    frame.setContentPane(new Container() {
        @Override
        public void paint(Graphics g) {
            super.paint(g);
            g.setColor(Color.GRAY);
            g.drawRect(x, y, width, height);
            g.setColor(Color.MAGENTA);
            g.drawArc(x, y, width, height, startAngle, arcAngle);
            g.setColor(Color.BLUE);
            g.drawLine((int) x1, (int) y1, (int) x2, (int) y2);
            g.drawLine((int) x0, (int) y0, (int) x3, (int) y3);
        }
    });

    frame.setSize(817, 839);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}