$('#Welcome_points>.Wpoint')[Math.floor(Math.random() * $('#Welcome_points>.Wpoint').length)].addClass('a1');
$('#Welcome_points>.Wpoint')[Math.floor(Math.random() * $('#Welcome_points>.Wpoint').length)]
$( $('#Welcome_points>.Wpoint')[Math.floor(Math.random() * $('#Welcome_points>.Wpoint').length)] ).addClass('a1');