Finding the sum of the interior angles of a regular n-gon...

(n-2)180 degrees.

So lets apply this! 

Say you have a decagon...




Decagon has 10 sides, so lets plug that into the formula for "n."

(10-2)180 

10-2= 8

180 times 8= 1440.

So to find each interior angle you...

1440/ 10= 144 degrees. 

We can conclude each interior angle is 144 degrees.