$.fn.drawArrow = function(x, y, l, angle, opts)
{
var elm = AssignOpts($(this), opts);

var x1 = x+Math.floor(l*Math.cos(angle));
var y1 = y-Math.floor(l*Math.sin(angle));
elm.drawLine(elm, x, y, x1, y1);

var x2 = x1+Math.floor(l/2*Math.cos(angle-5*Math.PI/6));
var y2 = y1-Math.floor(l/2*Math.sin(angle-5*Math.PI/6));
elm.drawLine(elm, x2, y2, x1, y1);

var x3 = x1+Math.floor(l/2*Math.cos(angle+5*Math.PI/6));
var y3 = y1-Math.floor(l/2*Math.sin(angle+5*Math.PI/6));
elm.drawLine(elm, x3, y3, x1, y1);

return $(this);
}

example:
$("#myCanvas").drawArrow(500, 500, 30, (Math.PI/4), {color:'red', stroke: 3});
start point:(x,y)
length of arrow:l
angle:angle from x-axis