DropDownList的SelectedIndexChanged事件中怎么调用这段Js代码？

DropDownList的SelectedIndexChanged事件中如何调用这段Js代码？？
Js代码如下：
function   Sum()
                {
        document.getElementById( "TextBox3 ").value=Number(document.getElementById( "TextBox2 ").value)+Number(document.getElementById( "TextBox1 ").value);              
                }


事件：
protected   void   ddlcSaleMode_SelectedIndexChanged(object   sender,   EventArgs   e)
{
    //这里面该怎么写？？？调用Sum()
}


------解决方案--------------------------------------------------------
ClientScript.RegisterStartupScript(this.GetType(), " ", " <script> Sum() </script> ");
------解决方案--------------------------------------------------------
Response.write( " <script> Sum(); </script> ");