Derivation of Thevinin Equivalent Circuit Rext Solution ABOVBE CIRCUIT SHOWN THAT we have to apply thevenini theorem by considering Rext as OPEN BRANCH TERMINAL THAT MEANS RL = Rext hence from the figure it is clear that Vth ,theveninin voltage = vR IF Rext is open branch terminal Rth = 0 becoz after short circuit the volagae sources we get R and short circuit branch in parallel hence Rth = 0 CURRENT FGLOWING ACROSS Rexty = Vth / (Rth + RL) = vR/ Rext.