canvas怎么画一个圆形？

canvas画一个圆形效果如下：

<canvas id="canvas_img1">your browser does not support the canvas tag.</canvas>
<script type="text/javascript">
var canvas = document.getElementById('canvas_img1');
canvas.width = '250';
canvas.height = '100';
var ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.arc(50,50,50,0,Math.PI*2,true);
ctx.strokeStyle = '#00FF00';
ctx.fillStyle = '#00FF00';
ctx.fill();//实心
ctx.beginPath();
ctx.arc(200,50,50,0,Math.PI*2,true);
ctx.strokeStyle = '#00FF00';
ctx.fillStyle = '#00FF00';
ctx.stroke();//空心
</script>