Geometry:

Formulas about Polygons

These formulas may be helpful when you are working with triangles, squares, pentagons, hexagons, etc. However, they will not be very helpful concerning disappearing parrots ("Polly gone"...sorry, lame pun).

• The sum of all the (interior) angles in any n-sided polygon is:
  180(n - 2) degrees
• In particular, you should know off the top of your head that the sum of the angles in a triangle (plug n=3 into the above formula) is 180 degrees. You can use this property to find the measure of an angle in a triangle, given the measures of the other two angles.
• So, in a regular n-sided polygon, the measure of each angle can be found by dividing the first formula by n. In other words, each angle of a regular n-sided polygon measures
  degrees.
• The number of diagonals in an n-sided polygon is
  .
  See if you can work out why this formula works!