Question

Suppose we have $10points$ in a plane forming a decagon.

How many diagonals are there is the decagon.

Answer 1

Formula for calculating number of diagonals of any polygon of $n$ sides $=\frac{n\times (n-3)}{2}$

So here it's a decagon, that is a 10 sided polygon, So $n=10$.

Simply plug value of n into the formula , you get:

$=\frac{10\times (10-3)}{2}$

$=35$