*{
    margin:0;
    padding:0;
}
.header{
    display: flex;
    flex-direction: row;
    height: 13vh;
}
.headerImage {
    position: absolute;
    left: 0;
    width: 12.5vw;
    height: 5vw;
    margin: 1.5vh 0 1.5vh 0;
}
.nav{
    position: absolute;
    left: 25vw;
    width:70vw;
    height: 13vh;
    margin:0 auto;
}
ul{
    list-style:none;
    display: flex;
    flex-direction: row;
    justify-content: space-around;
}
ul li{
    float:left;
    align-items: center;
    text-align:center;
    line-height: 13vh;
}

a{
    text-decoration:none;
    color:#777777;
    font-size: 15px;
    font-weight: 400;
    display:block;
    width:10vw;
}
a:hover{
    color:#51B9FC;
}
ul li ul li{
    float:none;
    line-height: 5vh;
}
ul li ul li a{
    color: #000000;
    display: block;
    width: 10vw;
}
ul li ul{
    display:none;
    padding-top:1vh;
    background-color:#EEEEEE;
    padding-bottom: 1vh;
}
/*为了兼容IE7设置的CSS样式，但是又必须写在a:hover前面*/
ul li ul li a:link,ul li ul li a:visited{
    background-color:#EEEEEE;
}
ul li ul li a:hover{
    background-color: #2C99E6;
    color: #000000;
}
.free{
    color: #ffffff;
    line-height: 2.7em;
    padding-left:0.7vw;
    padding-right:0.7vw;
    background-color: #4BB8FF;
    border-radius: 30px;
    margin-top:6.5vh;
    transform: translate(0, -50%)
}
.free:hover{
    color: #ffffff;
}
ul li:hover{
    color:#51B9FC;
}