I presume you know that you cannot divide by 0 so where is ? Also, you cannot take the square root of a negative number. A fraction is positive if both numerator and denominator are positive or if both numerator and denominator are negative. So if x> 0 where is [itex]4- x^2[/itex] also positive? If x< 0 is where is [itex]4- x^2[/itex]?