How to find diagonals....what is formula?


And also explain how to use
And fast...

Dear Student,
Answer :

We know " The diagonal of a polygon is a line segment linking two non-adjacent vertices. "

And

Also, there is obviously no diagonal from a vertex back to itself.

And we can find number of diagonals of any Polygon by this formula 

Total number of diagonals  = $\frac{n \left( n - 3 \right)}{2}$ 

Here n  =  Number of sides of polygon 

As we want to find number of diagonals of Decagon .

We know decagon have 10 sides so ,

Total number of diagonals in decagon  = $\frac{10 \left( 10 - 3 \right)}{2} = 5 \times 7$  =  35 

Regards