# Vector.js

Vector is an open source javascript library for creating interactive graphics. View the repository, run the tests, or meet the team.

# Circle Defined By Three Points Open in Sandbox

## Description

An interactive that demonstrates how a circle can be uniquely described by thee points.

## Script

``````/**
* @title Circle Defined By Three Points
* @description An interactive that demonstrates how a circle can be uniquely described by thee points.
* @tags [math]
* @date July 8, 2019
* @author Kurt Bruns
*/
import { Interactive } from '../../index.js';
export default function main(id) {
// Initialize the interactive
let interactive = new Interactive(id);
interactive.border = true;
interactive.width = 600;
interactive.height = 300;
interactive.originX = interactive.width / 2;
interactive.originY = interactive.height / 2;
;
// Create three control points
let c1 = interactive.control(-70, -80);
let c2 = interactive.control(100, 0);
let c3 = interactive.control(0, 100);
// Create a circle
let circle = interactive.circle(0, 0, 0);
circle.style.fill = '#f8f8f8';
circle.style.stroke = '#333333';
circle.update = function () {
// Solve for general form coefficients
let A = c1.x * (c2.y - c3.y) - c1.y * (c2.x - c3.x) + c2.x * c3.y - c3.x * c2.y;
let B = (c1.x * c1.x + c1.y * c1.y) * (c3.y - c2.y) + (c2.x * c2.x + c2.y * c2.y) * (c1.y - c3.y) + (c3.x * c3.x + c3.y * c3.y) * (c2.y - c1.y);
let C = (c1.x * c1.x + c1.y * c1.y) * (c2.x - c3.x) + (c2.x * c2.x + c2.y * c2.y) * (c3.x - c1.x) + (c3.x * c3.x + c3.y * c3.y) * (c1.x - c2.x);
let D = (c1.x * c1.x + c1.y * c1.y) * (c3.x * c2.y - c2.x * c3.y) + (c2.x * c2.x + c2.y * c2.y) * (c1.x * c3.y - c3.x * c1.y) + (c3.x * c3.x + c3.y * c3.y) * (c2.x * c1.y - c1.x * c2.y);
// Calculate center point and radius
this.cx = -B / (2 * A);
this.cy = -C / (2 * A);
this.r = Math.sqrt((B * B + C * C - 4 * A * D) / (4 * A * A));
};
circle.update();
// Create a display dot at the center of the circle
let dot = interactive.circle(0, 0, 3);
dot.root.style.fill = '#333333';