A strange side length

Geometry Level 4

A regular polygon inscribed in a unit circle has a side length of 2 2 + 2 + 2 + 3 . \large \sqrt{2-\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{3}}}}} .

Find the number of sides of the polygon.


The answer is 96.

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1 solution

Karim Fawaz
Jun 21, 2016

Let the angle between 2 radii joining the centre of the circle to 2 adjacent vertices of the polygon be x.

Applying the cosine rule we get: ( s i d e l e n g t h ) 2 = 1 2 + 1 2 2 ( 1 ) ( 1 ) c o s ( x ) (side length)^{2} = 1^{2} + 1^{2} - 2 (1) (1) cos(x)

( 2 2 + 2 + 2 + 3 ) 2 (\sqrt{2-\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{3}}}}})^{2} = 2 - 2 cos(x)

( 2 2 + 2 + 2 + 3 ) (2-\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{3}}}}) = 2 - 2 cos (x)

2 cos (x) = ( 2 + 2 + 2 + 3 ) (\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{3}}}})

Squaring both sides we get:

4 c o s 2 ( x ) cos^{2} (x) = ( 2 + 2 + 2 + 3 ) (2+\sqrt{2+\sqrt{2+\sqrt{3}}})

4 c o s 2 ( x ) cos^{2} (x) - 2 = ( 2 + 2 + 3 ) (\sqrt{2+\sqrt{2+\sqrt{3}}})

2 cos (2x) = ( 2 + 2 + 3 ) (\sqrt{2+\sqrt{2+\sqrt{3}}})

Squaring both sides again we get:

4 c o s 2 ( 2 x ) cos^{2} (2x) = 2 + 2 + 3 2+\sqrt{2+\sqrt{3}}

4 c o s 2 ( 2 x ) cos^{2} (2x) - 2 = 2 + 3 \sqrt{2+\sqrt{3}}

2 cos (4x) = 2 + 3 \sqrt{2+\sqrt{3}}

Squaring both sides again we get:

4 c o s 2 ( 4 x ) cos^{2} (4x) = 2 + 3 2+\sqrt{3}

4 c o s 2 ( 4 x ) cos^{2} (4x) - 2 = 3 \sqrt{3}

2 cos (8x) = 3 \sqrt{3}

cos (8x) = 1 2 3 \frac{1}{2} \sqrt{3}

8x = 30 degrees

x = 30/8 degrees

Number of sides = 360 30 / 8 \frac{360}{30/8} = ( 360 ) ( 8 ) 30 \frac{(360)(8)}{30} = 96 sides

A n s w e r = 96 Answer = \boxed{96}

Moderator note:

Good approach to solve for the angle.

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