?<div?class="nav">
				<div?class="nav_left">
				</div>
				<div?class="nav_mid">
					<div?class="nav_mid_left">
						<ul>
							<li><a?href="#">首頁</a></li>
							<li><a?href="list.html">關于慕課</a></li>
							<li><a?href="list.html">新聞動態</a></li>
							<li><a?href="list.html">課程中心</a></li>
							<li><a?href="list.html">在線課程</a></li>
							<li><a?href="list.html">人才招聘</a></li>
						</ul>
					</div>
					<div?class="nav_mid_right">
						<form?action=""?method="post">	
							<input?type="text"?value=""/>
						</form>
					</div>
				</div><!--navmid結束-->
				<div?class="nav_right">
				</div>
			</div><!--導航結束-->
		</div><!--wrap結束????????????????-->

/*導航區域*/
.nav{
	height:40px;
}
.nav_left{
	width:10px;
	background:?url(../img/nav_left.jpg)?no-repeat;
	float:?left;
	height:?40px;
}
.nav_mid{
	width:?980px;
	background:?url(../img/nav_bg.jpg)?repeat-x;
	float:?left;
	height:40px;
}
.nav_right{
	width:?10px;
	background:?url(../img/nav_right.jpg)?no-repeat;
	float:left;
	height:40px;
}

這個不能對齊 是什么原因呢？