For a regular polygon with 'n' sides: External angle = 360/n For a regular polygon with 'n' sides: Internal angle = 180 - 360/n Polygon: Number of sides: External angles: Internal angles: Triangle 3 120 60 Square 4 90 90 Pentagon 5 72 108 Hexagon 6 60 120 Heptagon 7 51.4 128.6 Octagon 8 45 135 Nonagon 9 40 140 Decagon 10 36 144 Undecagon 11 32.7 147.3 Dodecagon 12 30 150 Special types of triangle Sides: Equilateral - All 3 sides equal and all 3 angles equal (to 60o). Isosceles - Two sides the same and the angles at the bottom of those sides are equal. Scalene - No sides or angles equal. Angles: Acute-angled - All angles are acute (less than 90o). Right-angled - One angle is 90o. Obtuse-angled - One angle is obtuse (greater than 90o). Quadrilaterals: Square - All sides equal, all angles 90o, 4 lines of symmetry, rotation symmetry of order 4. Rectangle - Two pais of equal and parallel sides, all angles 90o, two lines of symmetry, rotation symmetry of order 2. Rhombus - All sides equal, opposite sides parallel, two lines of symmetry, rotation symmetry of order 2. (Basically a square leaning over. Sometimes referred to as a diamond! Parallelogram - Two pairs of equal and parallel sides, opposite angles equal, no lines of symmetry, rotation symmetry of order 2. (Basically a rectangle leaning over.). Trapezium - One pair of parallel sides, no symmetry. Kite - Two pairs of equal sides next to each other, one line of symmetry, no rotation symmetry, string (only joking!). Circles Circumference = 2π r Or, if you want to use the diameter: Circumference = π d Area = π r2