Returns the arctangent, or inverse tangent, of the specified x and ycoordinates. The arctangent is the angle from the xaxis to a line containing the origin (0, 0) and a point with coordinates (x_num, y_num). The angle is given in radians between pi and pi, excluding pi.
Syntax
ATAN2(x_num,y_num)
X_num is the xcoordinate of the point.
Y_num is the ycoordinate of the point.
Remarks

A positive result represents a counterclockwise angle from the xaxis; a negative result represents a clockwise angle.

ATAN2(a,b) equals ATAN(b/a), except that a can equal 0 in ATAN2.

If both x_num and y_num are 0, ATAN2 returns the #DIV/0! error value.

To express the arctangent in degrees, multiply the result by 180/PI( ) or use the DEGREES function.
Examples
Formul a 
Description (Result) 
=ATAN2(1, 1) 
Arctangent of the point 1,1 in radians, pi/4 (0.785398) 
=ATAN2(1, 1) 
Arctangent of the point 1,1 in radians, 3*pi/4 (2.35619) 
=ATAN2(1, 1)*180/PI() 
Arctangent of the point 1,1 in degrees (135) 
=DEGREES(ATAN2(1, 1)) 
Arctangent of the point 1,1 in degrees (135) 