Adam,
The imaginary number i is used in Electrical Engineering just as a tool when dealing with signal phases. i is very useful specially when you want to calculate the power in a system where the current I and the voltage V are out of phase. Remember, i doesn't have a real meaning it's just a mathematical tool.
Now, the formula that you provided it is just a simple manipulation of numbers. If you substract N from both sides and simplify you would end up with X*N=X*N. It is just a simple algebra manipulation but it is good you caught it.




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