Consider a regular polygon with 10 sides. What is the number of triangles that can be formed by joining the vertices which have no common side with any of the sides of the polygon?

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NDA 02/2021: Maths Previous Year paper (Held On 14 Nov 2021)
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  1. 25
  2. 50
  3. 75
  4. 100

Answer (Detailed Solution Below)

Option 2 : 50
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Detailed Solution

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Formula used:

Triangle having no side common with those of the polygon = 

\(\rm \frac{n(n-4)(n-5)}{6}\)

n is the sides of the polygon.

Calculation:

Here n = 10

Triangle having no side common with those of the polygon

\(\rm \frac{10(10-4)(10-5)}{6}\)

\(\rm \frac{10\times 6\times 5}{6}\)

= 50

∴ The number of triangles is 50. 

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