如图电路,电源电压恒定不变.当$S_{1}$闭合、$S_{2}$断开时,电压表的示数为$3V$；当$S_{1}$断开,$S_{2}$闭合时,电压表的示数$9V$,则电源电压$U=\_\_\_\_\_\_V$,电阻$R_{1}$:$R_{2}=$______.