window.onload=window.onresize=function(){  
if(document.getElementById("box_left").clientHeight<document.getElementById("box_con").clientHeight){  
document.getElementById("box_left").style.height=document.getElementById("box_con").offsetHeight+"px";  
document.getElementById("box_right").style.height=document.getElementById("box_con").offsetHeight+"px";  
 }  
else{  
document.getElementById("box_left").style.height=document.getElementById("box_right").offsetHeight+"px";  
document.getElementById("box_right").style.height=document.getElementById("box_con").offsetHeight+"px";  
 }  
 }