atan
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header <math.h>
  | ||
| float       atanf( float arg ); | (since C99) | |
| double      atan( double arg ); | ||
| long double atanl( long double arg ); | (since C99) | |
Computes arc tangent of arg
[edit] Parameters
| arg | - | floating point value | 
[edit] Return value
Arc tangent of arg in radians in the range of [-π/2; π/2] radians.
[edit] See also
| arc tangent, using signs to determine quadrants (function) | |
| computes arc sine (arcsin(x)) (function) | |
| computes arc cosine (arccos(x)) (function) | |
| computes tangent (tan(x)) (function) | |