## Algebra and Trigonometry 10th Edition

Published by Cengage Learning

# Chapter 11 - 11.6 - Counting Principles - 11.6 Exercises - Page 825: 71

20 diagonals.

#### Work Step by Step

We must select two vertices among eight vertices. The order in which they are selected is not important. So, we have $_8C_2$ different combinations: $_8C_2=\frac{8!}{(8-2)!~2!}=\frac{8\times7\times6!}{6!~2!}=28$ But, among those 28 combinations, 8 are sides. $Diagonals=28-8=20$

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