double exp(double x)
Mathematical Functions: math.h
double atan(double x)
The return value is the inverse tangent
or arctangent of y/x.
The return value will be expressed in radians
and lie in the interval 
.
The advantage of atan2() over atan() is that, because it is given separate access to the two original (rectangular) co-ordinates, it is able to distinguish beween angles in the left and right half planes (which is not possible if only y/x is passed in, as is the case with atan()).