The latus rectum of an ellipse

						
JXG.Options.text.useKatex = true;
JXG.Options.axis.ticks.majorHeight = 10;
JXG.Options.axis.ticks.insertTicks = false;
JXG.Options.axis.ticks.ticksDistance = 100;
var board = JXG.JSXGraph.initBoard('box1', {
boundingbox:[-10, 10, 10, -10],showNavigation:false, showcopyright: false, axis: true,
});

var F1 = board.create('point', [-6,0],{name:'F1',fixed:true, label:{offset:[-45,10]}});
var F2 = board.create('point', [function(){return -F1.X()},function(){return F1.Y();}],{name:'F2',label:{offset:[-35, 10]}});
var p1 = board.create('point', [1,4],{name:'',size:0,fixed:true});
var el = board.create('ellipse',[F1,F2,p1],{strokeColor:'red'});
//var g1 = board.create('glider', [2.0, 1.5, el],{name:'P'});
//var l1 = board.create('segment', [F1, g1],{strokeWidth:1});
//var l2 = board.create('segment', [F2, g1],{strokeWidth:1});
var x1 =  board.create('point', [1,0],{visible:false});
var x2 =  board.create('point', [5,0],{visible:false});
var cx = board.create('line', [x1,x2],{visible:false});
var y1 =  board.create('point', [0,1],{visible:false});
var y2 =  board.create('point', [0,-1],{visible:false});
var cy = board.create('line', [y1,y2],{visible:false}); 
var i1 = board.create('intersection', [cx, cy],{visible:false});
var vy1 = board.create('intersection', [cy, el],{visible:false});
var vy2 = board.create('intersection', [cy, el,1],{visible:false});
var vx1 = board.create('intersection', [cx, el],{visible:false});
var vx2 = board.create('intersection', [cx, el,1],{visible:false});
var l1 = board.create('segment', [i1, vx1],{strokeWidth:1});
var a = l1.L();
var l2 = board.create('segment', [i1, vy1],{strokeWidth:1});
var b = l2.L(); 
var l3 = board.create('segment', [i1, F1],{strokeWidth:1});
var c = l3.L(); 
var p2 = board.create('point', [a*a/c,0],{visible:false});
var pl1 = board.create('parallel', [cy, p2],{strokeWidth:1});
var pl2 = board.create('parallel', [cy, F2],{visible:false});
var i2 = board.create('intersection', [pl2, el],{name:'(c,l)',size:0,label:{offset:[-20,25]}});
var i3 = board.create('intersection', [pl2, el,1],{name:'(c,-l)',size:0,label:{offset:[-20,-25]}});
var l4 = board.create('segment', [i2, i3],{strokeWidth:1});

var pl3 = board.create('parallel', [cy, F1],{visible:false});
var i5 = board.create('intersection', [pl3, el],{visible:false});
var i6 = board.create('intersection', [pl3, el,1],{visible:false});
var l5 = board.create('segment', [i5, i6],{strokeWidth:1});


var txt1 = board.create('text',[a*a/c+1,0.5,function(){return 'Directrix';}],{display:'internal',rotate:function(){
ang =  90 ; return ang;
}, fontSize:16 });