extjs汇总值的列与前2列相乘

,{
    id: \'result_id\',
    header: \" \",
    sortable: true,
    dataIndex: \'result\',            
    width: 100,
    renderer: function (value, style, record, row, col, store, grid) {
      return Ext.util.Format.number(record.get(\'prew1\') * record.get(\'prew2\'), \'0.00\');
    },  
    summaryType: \'sum\',
    summaryRenderer: function(v) {
        return  Ext.util.Format.number(v, \'0.00\');
    }  

summaryType:function (values) {
    var result = [], retval;

    Ext.Array.forEach(values, function (record){
        result.push(record.get(\'area\') * record.get(\'yield\'));
    });       
    retval = Ext.Array.mean(result); 
    return Ext.util.Format.number(retval, \'0.00\');
},