@@ -0,0 +1,945 @@
+package stdlibpack;%0A%0Apublic class Charge %7B%0A    private double rx, ry;   // position%0A    private double q;        // charge%0A%0A    public Charge(double x0, double y0, double q0) %7B%0A        rx = x0;%0A        ry = y0;%0A        q  = q0;%0A    %7D%0A%0A    public double potentialAt(double x, double y) %7B%0A        double k = 8.99e09;%0A        double dx = x - rx;%0A        double dy = y - ry;%0A        return k * q / Math.sqrt(dx*dx + dy*dy);%0A    %7D%0A%0A    public String toString() %7B%0A        return q + %22 at %22 + %22(%22 + rx + %22, %22 + ry + %22)%22;%0A    %7D%0A%0A    public static void main(String%5B%5D args) %7B%0A        double x = Double.parseDouble(args%5B0%5D);%0A        double y = Double.parseDouble(args%5B1%5D);%0A        Charge c1 = new Charge(.51, .63, 21.3);%0A        Charge c2 = new Charge(.13, .94, 81.9);%0A        System.out.println(c1);%0A        System.out.println(c2);%0A        double v1 = c1.potentialAt(x, y);%0A        double v2 = c2.potentialAt(x, y);%0A        StdOut.println(v1+v2);%0A    %7D%0A%7D%0A

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