(2+Ra)/(Ra+R2) = 4/10
20+10Ra = 4Ra + 4R2
Ra = (4R2 - 20)/6

(2+Rb)/(Rb+R2) = 8/11
22+11Rb = 8Rb + 8R2
Rb = (8R2 - 22)/3

总电压V = 4/(2+Ra) * (2+Ra+R2) = 11/(R2+Rb) * (2+Rb+R2)
分别代入Ra和Rb，可算得R2等于8