Many circuits are composed of pieces that are resistors in series and pieces that are resistors in parallel. These pieces may be reduced to equivalent resistors.
Here, resistors R2, R3, and R4 are in parallel so they can be replaced by a single, equivalent resistor Req.
Redrawing a circuit may make it easier to understand.
Now it is clear that these resistors are in series.
What single equivalent resistor REQ replaces this bank of resistors in series?
Most strings of minature Christmas lights are made of bulbs that are connected in series. But each of those bulbs has a filament connected in parallel to another conductor in a way that this "shunt conductor" becomes part of the circuit only when the filament burns out. (Serway's text has a good, detailed description of this).
What happens if one of the filaments burns out?
What happens if one of the bulbs is removed?
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(c) Doug Davis, 2002; all rights reserved