The sum of interior angles of a polygon having n sides is (2n – 4) right angles.
Thus, each interior angle of the polygon = ((2n – 4) right angles) / n.
Triangle
(2n - 4) right angles = (2 × 3 - 4) × 90° = (6 - 4) × 90° = 2 × 90° = 180°
each interior angle = 180° / 3 = 60°
Pentagon
(2n - 4) right angles = (2 × 5 - 4) × 90° = (10 - 4) × 90° = 6 × 90° = 540°
each interior angle = 540° / 5 = 108°
Octagon
(2n - 4) right angles = (2 × 8 - 4) × 90° = (16 - 4) × 90° = 12 × 90° = 1080°
each interior angle = 1080° / 8 = 135°