R3并联在由R1和R2串联的电路上.已知其各电阻值分别为R1,R2,R3.求R1与R3消耗的电功率之比再描述下图,R1和R2串联,R3并联在这两个的串联上

P1/P3=(U1^2/R1)/(U3^2/R3)=(U1^2/U3^2)(R3/R1)=[R1^2/(R1+R2)^2](R3/R1)=R1R3/(R1+R2)^2

设并联两端电压为V
W1=V^2*R1/(R1+R2)^2
W3=V^2/R3
R1/R3=R1*R3/(R1+R2)^2

令并联支路电压为U
P3=U^2/R3
I1 = U/(R1+R2)
P1=I1^2R1=U^2R1/(R1+R2)^2
P1：P3 = { U^2R1/(R1+R2)^2 } ：{ U^2/R3 } = R1R3/(R1+R2)^2