Reference Node
In circuits, we usually label a node as the reference node also called ground and define the other node voltages with respect to this point. The reference node has a potential of 
by definition. The following symbol is used to indicate the reference node:
As mentioned, the selection of the reference node is arbitrary. However, a wise selection can make the solving easier. As a general rule, it is usually chosen to be
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and 
. (Hint: there is no need to use nodal analysis; voltages between nodes can be easily found by the voltage sources.)
