Polygons: Calculating the Number of Diagonals

In this video, I demonstrate how to calculate the number of diagonals in a polygon.

[asiangal]

Hi may i know the formula for finding the no. Of triangles formed by diagonals in a polygon

[luispanameno]

dang you came in clutch I smashed that like button

[dimasgomez]

We take the n number of vertices times the possible diagonals (n-3), then consider just the ones that are not duplicated. This gives us [n(n-3)]*1/2
For a ABCDEF hexagram, we have (repeated diagonals are in parentheses):
AC AD AE
BD BE BF
CE CF (CA)
DF (DA) (DB)
(EA) (EB) (EC)
(FB) (FC) (FD)

[wiseoldglitchakajuju]

Can you explain exactly how the formula works?

[lizph]

why do we need to subtract 3? why 3? 😁 thank you

[anjalikatyal]

there are 3 diagonals in a need to draw a vertex in the centre