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The number of diagonals that can be drawn by joining the vertices of an octagon is

1. $28$
2. $48$
3. $20$
4. None of the option

A diagonal is a line segment joining two vertices whereas those vertices are not sharing the same edge.

Therefore,

The number of diagonals that can be drawn by joining ‘n’ vertices =  (Total edges possible) – n

= C(n,2) – n

So for octagon the no. of diagonals possible = 20

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