Get content page field value
What do you mean by content page? if it is the node page and the fields name are field_number_a and field_number_b then install entity module and in the template.php and in hook_preprocess_node function and write this:

function YourTheme_preprocess_node(&amp;$var){
    if($var['node']-&gt;type == "YOUR_CONTENT_TYPE"){
          $wrapper = entity_metadata_wrapper('node',$var['node']);
          $a = $wrapper-&gt;field_number_a-&gt;value();
          $b = $wrapper-&gt;field_number_b-&gt;value();
          $var['total'] = $a + $b;
    }
}


Then in the node page you can use $total variable as the sum of the a and b