What is the number of diagonals of an octagon?

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NDA (Held On: 17 Nov 2019) Maths Previous Year paper
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  1. 48
  2. 40
  3. 28
  4. 20

Answer (Detailed Solution Below)

Option 4 : 20
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Detailed Solution

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Concept:

The no. of diagonals that can be drawn by joining the angular points of a polygon having n sides is given by: \({\;^n}{C_2} - n = \frac{{n \times \left( {n - 3} \right)}}{2}\)

Calculation:

Given: The polygon is an octagon ⇒ The no. of sides of the given polygon n = 8.

As we know that, no. of diagonals that can be drawn by joining the angular points of a polygon having n sides is given by: \({\;^n}{C_2} - n = \frac{{n \times \left( {n - 3} \right)}}{2}\)

\({\;^8}{C_2} - 8 = \frac{{8 \times 5}}{2} = 20\)
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