text: function doFirst(){
  	var x = document.getElementById('canvas');
  	var canvas = x.getContext("2d");
  
  	canvas.lineWidth = 5;
  	canvas.strokeStyle = 'red';
  
  	canvas.arc(150,150,80,0,Math.PI,false);
  	canvas.stroke();
  	
  	canvas.beginPath(); //才不會連在一起
  	canvas.arc(400,150,80,0,Math.PI,true);
  	canvas.stroke();
  
  	canvas.beginPath();
  	canvas.arc(150,400,80,0,2*Math.PI,true);
  	canvas.stroke();
  
  	canvas.beginPath();
  	canvas.arc(400,400,80,0.3*Math.PI,1.2*Math.PI,false);
  	canvas.stroke();
  }
  
  window.addEventListener('load', doFirst, false);