设电源电压为u，活动变阻器最大阻值为r；则ur/(r+r1)=4.5u(r/2)/(r/2+r1)=3=ur/(r+2r1)所以4.5(r+r1)=3(r+2r1),r=r1=30,所以u/2=4.5,u=9
R1 R2串联 在滑片改变时 电源电压不变 列方程在中点时 U=IR总=3/R2/2(R1+R2/2)B点时 U =IR总=4.5/R2(R1+R2)解得 U=9v R2=30Ω
