// Define the id of your board in BOARDID
var board = JXG.JSXGraph.initBoard(BOARDID, {
boundingbox: [-10, 10, 10, -10]
});
var a = board.create('slider', [[1, 9], [5, 9], [0, 1, 4]], {name: 'a'});
var c1 = board.create('curve', [
(phi) => a.Value() * Math.sqrt(phi),
[0, 0], 0, 8 * Math.PI
], {curveType: 'polar', strokewidth: 4});
var c2 = board.create('curve', [
(phi) => -a.Value() * Math.sqrt(phi),
[0, 0], 0, 8 * Math.PI
], {curveType: 'polar', strokewidth: 4});