点击页面会自动随机画出一个三角形！！！
<!DOCTYPE HTML>
<html>
<head><meta name="viewport" content="width=320,user-scalable=no"></head>
<body>
<canvas id="canvas" width="3000" height="3000"></canvas>
<script type="text/javascript">
var canvas=document.getElementById("canvas");
var cxt=canvas.getContext("2d");
canvas.onmousedown=function (event){
var x=event.x;
var y=event.y;
var r=Math.random()*5+5;
cxt.beginPath();
cxt.moveTo(x,y);
cxt.lineTo(x,y+r);
cxt.lineTo(x+r,y+r);
cxt.lineTo(x,y)
cxt.strokeStyle="black";
cxt.stroke();
}
</script>
</body>
</html>
截图如下：
