Therefore, the voltage across R3 of R 1 series is U, and the voltage across R3 is U3 = U× R3/(R1+R3);
In R2 series, the voltage across R4 is u, and the voltage across R4 is U4=U×R4/(R2+R4).
UAB = U3-U4 = UR3/(r 1+R3)-UR4/(R2+R4).
It can also be understood that if the lowest node is m, then U3=Uam and U4=-Umb.
Uab=Uam+Umb=U3-U4 .