用<canvas>标签绘制图形浏览器显示不出来

代码如下：

 <!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
    <title></title>
    <script type="application/javascript">
        function draw(){
            var canvas=document.getElementById('canvas');
            if(canvas.getContext){
                var ctx=canvas.getContext("2d");
                for(var i=0;i<2;i++){
                    for(var j=0;j<3;j++){
                        ctx.beginPath();
                        var x=10+j*30;
                        var y=10+i*30;
                        var radius=10;
                        var stratAngle=0;
                        var endAngle=Math.PI+(Math.PI*j)/2;
                        var anticlockwise=false;
                        ctx.arc(x,y,raidus,stratAngle,endAngle,anticlockwise);
                        ctx.stroke();
                    }               
                }
            }
        }


    </script>
</head>
<body onload="draw()">
<canvas id="canvas" width="200" height="200"></canvas>
</body>
</html>

我还找了另一个例子，照着敲的也显示不出来

raidus没有定义


<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title></title>
    <script>
        function draw(){
            var canvas = document.getElementById('canvas');
            var raidus=20///////////////////
            if(canvas.getContext){
                var ctx=canvas.getContext("2d");
                for(var i=0;i<2;i++){
                    for(var j=0;j<3;j++){
                        ctx.beginPath();
                        var x=10+j*30;
                        var y=10+i*30;
                        var radius=10;
                        var stratAngle=0;
                        var endAngle=Math.PI+(Math.PI*j)/2;
                        var anticlockwise=false;
                        ctx.arc(x,y,raidus,stratAngle,endAngle,anticlockwise);
                        ctx.stroke();
                    }
                }
            }
        }


    </script>
</head>
<body onload="draw()">
    <canvas id="canvas" width="200" height="200"></canvas>
</body>
</html>