var rad = Math.atan2(x2 - x1, y2 - y1);
var x3 = x1 + Math.sin(rad + 60 * (Math.PI / 180)) * r;
var y3 = y1 + Math.cos(rad + 60 * (Math.PI / 180)) * r;
var x4 = x1 + Math.sin(rad - 60 * (Math.PI / 180)) * r;
var y4 = y1 + Math.cos(rad - 60 * (Math.PI / 180)) * r;
drawCircle(x3, y3, 4, "#f00");
drawCircle(x4, y4, 4, "#f00");

var sqrt3 = Math.sqrt(3);
var cx = (x1 + x2) / 2;
var cy = (y1 + y2) / 2;
var x5 = cx - (y2 - y1) / 2 * sqrt3;
var y5 = cy + (x2 - x1) / 2 * sqrt3;
var x6 = cx + (y2 - y1) / 2 * sqrt3;
var y6 = cy - (x2 - x1) / 2 * sqrt3;
drawCircle(x5, y5, 2, "#0f0");
drawCircle(x6, y6, 2, "#0f0");

うん、あたまわるい