Answer 1

the logarithm function doesn't exist for x<=0, so you can expect this sort of behaviour.
specifically, what you're seeing here is that the division by zero you have in the (x-1)/x part is evaluated as a special floating point number, "-inf" (negative infinity), which is then combined with other values in a way that doesn't make sense mathematically but does make sense in the world of floats, to finally give you the number "-nan", which means "(negative) Not A Number"; basically this means you've done operations in a way that doesn't make sense in common algebra but instead of crashing the computer tries to make them make sense for as long as possible.