here is a visual guide. if you are moving along this number line from left to right, the numbers are increasing in value. if you are moving from right to left, the numbers are decreasing in value.
interior angle of a regular 18-gon. It is easier to calculate the exterior angle of a regular polygon of n-sides (n-gon) by the relation exterior angle = 360/n For a 18-gon, n=18, so exterior angle = 360/18=20 ° The value of each interior angle is therefore the supplement, or Interior angle = 180-20=160 degrees.
Naming of a 9-gon A polygon with 9 vertices is called a nonagon (in English) or enneagon (French ennéagone, but the English version is sometimes used) You had a good start with the correct answer.
Exterior angle of a 15-gon The exterior angle of a 15-gon can be calculated using the relation given in the first paragraph, namely Exterior angle = 360/15=24 degrees
