what is opposite side and adjacent side ?
If two sides of a polygon share a common vertex, then the sides are called adjacent sides.
Consider a polygon of 2n-sides (even number of sides). The opposite side is the nth side counting from the given side in either direction.
Consider the following octagon ABCDEFGH.
In this octagon, sides AB and BC share a common vertex B. So, AB and BC are adjacent sides.
There are 8 sides. In order to find opposite side of AB, count (8/2) = 4th side from any direction from AB (exclude AB). It is EF.
So, EF is the opposite side of AB.