# 18 degree angle

• Nov 27th 2009, 01:44 PM
wolfhound
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
• Nov 27th 2009, 01:52 PM
e^(i*pi)
Quote:

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
• Nov 27th 2009, 02:00 PM
wolfhound
Quote:

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?
• Nov 27th 2009, 02:13 PM
skeeter
Quote:

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.
• Nov 27th 2009, 02:39 PM
wolfhound
Quote:

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