It is the case, since it was a premise that x and y are both 1. So the two things you list are both 0, so they are equal, given the premise. So, the actual pro

It is the case, since it was a premise that x and y are both 1. So the two things you list are both 0, so they are equal, given the premise. So, the actual problem is dividing by zero. Your assumption that "we are dealing with algebra and not numerical values" is false because it completely ignores the "if x and y = 1" part.