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In FF6 you considered and hopefully explored the following procedures:
TO CIRCLE :CH
REPEAT 360 [ FD :CH RT 1]
END
TO CIRC :CH :ANG
REPEAT 360 [ FD :CH RT :ANG]
END
As a consequence different sized circles and some polygons may have resulted. Did any of you manage to draw a heptagon ($7$ sided)? A nonagon ($9$sided)? A endecagon($11$ sided)? $13$gon? Etc. etc....
Imagine walking around the outside of a pentagon....as you: go forward then turn, go forward then turn, go forward then turn, go forward then turn, ... finally go forward then turn. You should be back where you started... go on try it, convince yourself. In your journey you should have turned through $360^\circ$ .
Five times you turned through $72^\circ$ . N.B. $5 \times72 = 360$.
Hence:
For a pentagon  REPEAT 5 [ FD 45 RT 360/5]
For a heptagon  REPEAT 7 [FD 45 RT 360/7]
For a nonagon  REPEAT 9 [ FD 45 RT 360/9]

See the pattern?
So why not experiment?
Go on try:
TO POLY :N
REPEAT :N [FD 45 RT 360/:N]
END
Then try:
TO POLY :N :M
REPEAT :N [FD :M RT 360/:N]
END
Next: FF8