Confusing Parallel/Series Arrangements in Application of Thévenin's Theorem
The Thevenin equivalent resistance is one that looks to the circuit you want to replace. To find this value, all independent sources must be passivated.

Passivate ideal voltage source means replacing it by a short circuit. Passivate ideal current source means replacing it by an open circuit.

If the 5V supply is passivated in this specific case, the two resistors are connected in parallel, and here the value of the Thevenin equivalent resistance.







simulate this circuit &ndash; Schematic created using CircuitLab

ERRATA: In the schematic, "V2" must be \$V_{th}\$, Thevenin's equivalent voltage source.