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NDA 2019
Exam
NDA
Year
2019
Subject
Mathematics
Chapter
Permutation and Combination

NDA 2019 · Mathematics · Permutation and Combination

What is the number of diagonals of an octagon?

What is the number of diagonals of an octagon?

Options

  1. A48
  2. B40
  3. C28
  4. D20

Correct answer

D. 20

Step-by-step solution

∵ Numbers of diagonals =nC2−n∴ diagonals in octagon =8C2=−8=8×72−8=28−8=20\begin{aligned} & \because \text{ Numbers of diagonals }={ }^n C_2-n \\ & \therefore \text{ diagonals in octagon }={ }^8 C_2=-8 \\ & =\frac{8 \times 7}{2}-8 \\ & =28-8=20\end{aligned}
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