Angles in a semicircle are 90 degrees

						var board = JXG.JSXGraph.initBoard('box1', {
boundingbox: [-10, 10, 10, -10],showNavigation:false, showcopyright: false, axis: false,
});

var p1 = board.create('point', [0, 2.0],{name:'O',face:'',label:{offset:[-20,0]}});
var c1 = board.create('circle', [p1, 6],);
var g1 = board.create('glider', [1.0, 4.5, c1],{name:'',face:''});

var g2 = board.create('glider', [4.0, 2, c1],{name:'',face:''});

var l1 = board.create('line', [p1,g1],{visible: false});
var i1 = board.create('intersection', [c1, l1, 1],{name:'',face:''});
var l2 = board.create('segment', [i1,g1]);

var l3 = board.create('segment', [i1,g2]);
var a1 = board.create('angle', [g2, i1, g1],{radius:2,label:{offset:[-5,3]}});

var l4 = board.create('segment', [p1,g2]);

var a2 = board.create('angle', [p1, g1, g2],{radius:1.5});
var l5 = board.create('segment', [g1,g2]);
var a3 = board.create('angle', [g1, g2, p1],{radius:1.5,name:'β'});
var a4 = board.create('angle', [p1, g2, i1],{radius:2,name:'α'});