设甲乙的电阻分别为R1、R2,水加热到沸腾所需热量为Q
甲：220^2/R1*t1=Q(1)
甲乙串联：220^2/(R1+R2)*t2=Q (2)
甲乙并联：220^2*(R1+R2)/(R1*R2)*t3=Q (3)
(1)/(2),得R1*t1=(R1+R2)*t2
推出R1:R2=t2:(t1-t2) (4)
(1)/(3),得R1*t1=(R1*R2)/(R1+R2)*t3
所以t3=t1(R1+R2)/R2
将（4）代入上式 ,得
t3=t1*(t2+t1-t2)/(t1-t2)=t1^2/(t1-t2)
第一次回答问题啊 给分哦