attach file I need help number 12 only,thank
Presuming that R1 is the variable of interest, then the domain boils down to asking "what are the range of possible values for the resistance in resistor #1?" Express that range of values in proper notation and you are set. (ie, is it possible to have no resistor, a 5 Ohm resistor, a 200 Ohm resistor, etc.) For range, it boils down to asking "what is the range of possible values for the current?". Again, assuming R2 and R3 are fixed as given in question #9, consider the domain you gave, substitute the two endpoints for that domain, and that will give you the range. As a rule of electronics (and the formula), the more resistance you put on there, the lower the current will be, and vice-versa.
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