Product of two table feilds

Question

I have a table bellow

productId       productQty     productRate
1                         10           100
2                         20           200
3                         30           300    
4                         40           400
5                         50           500    


i need the out put is bellow 

productQty
-----------
(10*100)+
(20*200)+
(30*300)+
(40*400)+
(50*500)

in SQL Server 2005