//侧边定位
$(window).scroll(function(){
	var winTop = $(this).scrollTop();
	var main_left_top = $('.main_content').offset().top - 90;
	if( winTop >= main_left_top ){
		$('.main_left').addClass('main_left_fixed')
	}else{
		$('.main_left').removeClass('main_left_fixed')
	}
})