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Math Help - 18 degree angle

  1. #1
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    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
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  2. #2
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    e^(i*pi)'s Avatar
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    Quote Originally Posted by wolfhound View Post
    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
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  3. #3
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    Quote Originally Posted by e^(i*pi) View Post
    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?
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  4. #4
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    Quote Originally Posted by wolfhound View Post
    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.
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  5. #5
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    Quote Originally Posted by skeeter View Post
    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
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