An interior angle of a regular decagon is given by the formula (n-2) * 180 / n, where n is the number of sides. Since a decagon has 10 sides, the measure of each interior angle is (10-2) * 180 / 10 = 160 degrees.
The measure of each exterior angle of a regular polygon is given by the formula 360 / n, where n is the number of sides. So in the case of a decagon, each exterior angle measures 360 / 10 = 36 degrees.