Section 6: Functions93
In the HP-41C, angle θ is represented in the following manner:
To convert from rectangular x, y coordinates to polar (r, θ) coordinates (magnitude and angle, respectively):
  1. Key in the the y-coordinate.
  2. Press ENTER  .
  3. Key in the the x-coordinate.
  4. Execute R–P (rectangular to polar). Magnitude r is placed in the X-register and angle θ is placed in the Y-register. To display the θ value, press X Y .
When you execute the R–P function, the stack contents are changed...
  ...from this...  this.  
T  t
T  t
Z  z
Z  z
Y  y-coordinate
Y  angle θ
X  x-coordinate
X  magnitude r