What is the sum of angles in a octadecagon?

( n x 180 ) - 360

where

n is the number of sides.

In other words,

take the number of sides or your polygon and multiply that by 180.

then, subtract 360 from that result.

that is your answer.

An octadecagon has 18 sides.

( 18 x 180 ) - 360 = 2880

The sum of the interior angles of an octadecagon is 2880 degrees.