随窗口大小改变位置
获取屏幕宽度，resize：窗口大小改变时触发
function tishi(){
var one = document.body.clientWidth;
var wid = (one-1000)/2-50;
if(wid>=0){
$('.left-tishi').css('left',wid+'px');
}else{
$('.left-tishi').css('left','10px');
}}
tishi();
$(window).resize(function(){
tishi();
});