试用for、while、do...while三种形式计算1+2+3+...+20?

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<script language="javascript">
<!--

//用for计算1+2+3+....+20
   var num = 0;
   var total = 0;
   for(num = 0; num < 21; num++)
   {
      total = total+num;

   }
      document.write("--------- 总------: "+total);

//用while计算1+2+3+....+20
   var num = 0;
   var total = 0;   
   while( num < 21)
   {
      total=num+total; num++;

   }
      document.write("总和: "+total);

//用do...while计算1+2+3+....+20
   var num = 0;
   var total = 0;
   do   
   {
      total = total+num; num++;

   }
     while(num< 21 );
      document.write("--------- 总------: "+total);

-->
</script>

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