# Math Help - 18 degree angle

1. ## 18 degree angle

How can I form a 18 degree angle with just a ruler and compass
I know how to make a 45 degree angle by doing a random arch within 90 degree and then intersecting 2 archs..
but how can I get 18 degrees

2. Originally Posted by wolfhound
How can I form a 18 degree angle with just a ruler and compass
I know how to make a 45 degree angle by doing a random arch within 90 degree and then intersecting 2 archs..
but how can I get 18 degrees
$18 = \frac{360}{20}$ so draw 1/20 of a circle or 1/10 of a semicircle

3. Originally Posted by e^(i*pi)
$18 = \frac{360}{20}$ so draw 1/20 of a circle or 1/10 of a semicircle
Yes
But the question is how do I draw 1/10 of a semi circle?

4. Originally Posted by wolfhound
How can I form a 18 degree angle with just a ruler and compass
I know how to make a 45 degree angle by doing a random arch within 90 degree and then intersecting 2 archs..
but how can I get 18 degrees
I believe you need to construct a regular pentagon (directions can be found using a google search).

drawing in the diagonals of the pentagon forms the familiar 5-pointed star, each point is 36 degrees ... bisect that angle.

5. Originally Posted by skeeter
I believe you need to construct a regular pentagon (directions can be found using a google search).

drawing in the diagonals of the pentagon forms the familiar 5-pointed star, each point is 36 degrees ... bisect that angle.
Yes it worked
Thanks