The relationship between internal angles of a tringle and its external angle

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

var Inpt = [15,12,12,["A","C","B"],'SSS'];
var tri = board.create('triangle',Inpt,{D:"LT",'line':{withLabel:false,strokeColor:'blue',strokeWidth:2},'angle':{visible:false,strokeColor:'red',strokeWidth:1}});

var p1 = board.create('point', [tri.C.X()+0.6, tri.C.Y()],{face:'',name:''});
var l1 = board.create('segment', [p1, tri.C]);

var a1 = board.create('angle', [p1,tri.C, tri.B]);
var a2 = board.create('angle', [tri.B,tri.C, tri.A],{radius:1.5,name:'z°'});
var a3 = board.create('angle', [tri.A,tri.B, tri.C],{radius:1.5,name:'y°'});
var a4 = board.create('angle', [tri.C,tri.A, tri.B],{radius:1.5,name:'x°'});