Enter the function y1 = abs(ln(x)) on the "y =" screen, and graph it in a standard window. 

The screen shot at right shows what you (probably) get. If you are alert, you are asking yourself, "The graph in the first quadrant looks ok, but what's going on in the second quadrant? The domain of y = ln(x) is x > 0, so y = ln (x) should also be undefined when x = 0, right?" Yes, you are right  we don't expect to see the part of the graph in II. Read on. 

Set your calculator's angle to "degree", and regraph the function. 

This time, you get the "correct" graph. (If you got this graph initially, switch to radian mode to see the graph above.) Since we usually (always?) work in radian mode in AP Calculus, this is a little disturbing. What's going on? 

Go back to the screen again. Set Angle mode to RADIAN, and Complex Format to "2: Rectangular". 

Now, some theory. Do you remember from prealgebra (the "good old days"!) the equation involving the polar form of a complex number: If you let , this becomes: This is a form of the Euler equation , an equation that contains all of the important constants of elementary mathematics ("The Meaning of Life")! Suppose we try to evaluate ln(3) as a complex number: This is the result that the TI89 gives you if you evaluate ln(3). 

OK, so what about ln(3)? Well, in "complex number land", the " " symbol means "magnitude", not "absolute value"! Remember graphing complex numbers on the complex (Argand) plane? The magnitude of the complex number ln(3) is: which is a real number! (approximately 3.328)! 

The screen shot at right shows that the calculator is indeed plotting the point (3, 3.32815)! So, the calculator isn't wrong  it is just applying a little fancier math than we really want to see in this case. Remember that you can shut this off by switching to DEGREE . The reason that this works is that the identity is only valid if theta is in radians. 
