Okay, I'm solving to find the min and max of a function, f(x)= (a*x)/(x^2 + a^2). I found the derivative, set it to zero and concluded that the critical points are +/-a. So now I have to take a value a bit less than a and a value a bit greater than a to find out if the point is a min or max point. How do I do this if I have no numerical values for x or a? Can someone put some perspective on this? Thanks so much!