The description of the ATAN2 function in Microsoft Excel help does not seem
100% accurate to me. It states "ATAN2(a,b) equals ATAN(b/a), except that a
can equal 0 in ATAN2. "

I don't think that is accurate because ATAN2 takes into account the quadrant
in which z lies and has the range of -pi to pi (excluding -pi). This
special inverse tangent uses the sign of x and y to determine the quadrant,
and allows for answers in quandrant 2 (-x, +y) and quadrant 3
(-x,-y) whereas the standard arctan is restricted to quadrants 1 and 4
(range of -pi/2 to +pi/2).

There are cases in which ATAN(y/x) does not equal ATAN2(x,y), other than
when x = 0. For example, when x = -5 and y =-15, ATAN(-15/-5) = 1.249 and
ATAN2(-5,-15) = -1.89

I'm not a math expert, but this description seems misleading to me. I
suggest a description similar to the C++ ATAN2(x,y) function "The atan2()
function computes the arc tangent of y/x, using the signs of the arguments to
compute the quadrant of the return value. "