I am trying to reteach myself basic circuit theory using "Practical Electronics for Inventors 3rd Edition" by Scherz and Monk.
I am working an example in which it asks to calculate the voltage difference between a variety of points (I have included a diagram below. Please note that I could not find the typical ground symbol I'm used to , so I had to use the one provided under the name GND). The example asks to find the voltage difference between points: A and C (denoted as V[sub]AC[/sub]), B and D, A and D, and B and C. Now the book lists the voltage difference between A and C as plus +3 volts and the voltage difference between A and D as +12 V, then turns around and lists the voltage between B and C as +9 volts.
I don't see how they are getting the figure for the voltage between B and C (I keep calculating -9V by the way the other two voltages were calculated).