S1闭合、S2断开，只有R1，这是保温过程，P温=P1=U²/R1=U²/(9R2).S1S2闭合,加热状态。P热=P1+P2=U²/(9R2)+U²/R2=(10/9)U²/R2=500wP2=U²/R2=450wP温=P1=U²/R1=U²/(9R2)=50w
