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20 terms

triangle

3 sided polygon

quadrilateral

4 sided polygon

pentagon

5 sided polygon

hexagon

6 sided polygon

septagon

7 sided polygon (starts with s)

heptagon

7 sided polygon (starts with h)

octagon

8 sided polygon

nonagon

9 sided polygon

decagon

10 sided polygon

hendecagon

11 sided polygon

dodecagon

12 sided polygon

triskaidecagon

13 sided polygon

tetrakaidecagon

14 sided polygon

pentakaidecagon

15 sided polygon

hexakaidecagon

16 sided polygon

heptakaidecagon

17 sided polygon

octakaidecagon

18 sided polygon

eneeakaidecagon

19 sided polygon

icosagon

20 sided polygon

(n(n-3))/2

how to get diagonals